mfd: rn5t618: Make restart handler atomic safe

mfd: rn5t618: Make restart handler atomic safe

The restart handler is executed during the shutdown phase which is
atomic/irq-less. The i2c framework supports atomic transfers since
commit 63b96983a5dd ("i2c: core: introduce callbacks for atomic
transfers") to address this use case. Using i2c regmap in that
situation is not allowed:

[  165.177465] [ BUG: Invalid wait context ]
[  165.181479] 5.8.0-rc3-00003-g0e9088558027-dirty #11 Not tainted
[  165.187400] -----------------------------
[  165.191410] systemd-shutdow/1 is trying to lock:
[  165.196030] d85b4438 (rn5t618:170:(&rn5t618_regmap_config)->lock){+.+.}-{3:3}, at: regmap_update_bits_base+0x30/0x70
[  165.206573] other info that might help us debug this:
[  165.211625] context-{4:4}
[  165.214248] 2 locks held by systemd-shutdow/1:
[  165.218691]  #0: c131c47c (system_transition_mutex){+.+.}-{3:3}, at: __do_sys_reboot+0x90/0x204
[  165.227405]  #1: c131efb4 (rcu_read_lock){....}-{1:2}, at: __atomic_notifier_call_chain+0x0/0x118
[  165.236288] stack backtrace:
[  165.239174] CPU: 0 PID: 1 Comm: systemd-shutdow Not tainted 5.8.0-rc3-00003-g0e9088558027-dirty #11
[  165.248220] Hardware name: Freescale i.MX6 SoloLite (Device Tree)
[  165.254330] [<c0112110>] (unwind_backtrace) from [<c010bfa0>] (show_stack+0x10/0x14)
[  165.262084] [<c010bfa0>] (show_stack) from [<c058093c>] (dump_stack+0xe8/0x120)
[  165.269407] [<c058093c>] (dump_stack) from [<c01835a4>] (__lock_acquire+0x81c/0x2ca0)
[  165.277246] [<c01835a4>] (__lock_acquire) from [<c0186344>] (lock_acquire+0xe4/0x490)
[  165.285090] [<c0186344>] (lock_acquire) from [<c0c98638>] (__mutex_lock+0x74/0x954)
[  165.292756] [<c0c98638>] (__mutex_lock) from [<c0c98f34>] (mutex_lock_nested+0x1c/0x24)
[  165.300769] [<c0c98f34>] (mutex_lock_nested) from [<c07593ec>] (regmap_update_bits_base+0x30/0x70)
[  165.309741] [<c07593ec>] (regmap_update_bits_base) from [<c076b838>] (rn5t618_trigger_poweroff_sequence+0x34/0x64)
[  165.320097] [<c076b838>] (rn5t618_trigger_poweroff_sequence) from [<c076b874>] (rn5t618_restart+0xc/0x2c)
[  165.329669] [<c076b874>] (rn5t618_restart) from [<c01514f8>] (notifier_call_chain+0x48/0x80)
[  165.338113] [<c01514f8>] (notifier_call_chain) from [<c01516a8>] (__atomic_notifier_call_chain+0x70/0x118)
[  165.347770] [<c01516a8>] (__atomic_notifier_call_chain) from [<c0151768>] (atomic_notifier_call_chain+0x18/0x20)
[  165.357949] [<c0151768>] (atomic_notifier_call_chain) from [<c010a828>] (machine_restart+0x68/0x80)
[  165.367001] [<c010a828>] (machine_restart) from [<c0153224>] (__do_sys_reboot+0x11c/0x204)
[  165.375272] [<c0153224>] (__do_sys_reboot) from [<c0100080>] (ret_fast_syscall+0x0/0x28)
[  165.383364] Exception stack(0xd80a5fa8 to 0xd80a5ff0)
[  165.388420] 5fa0:                   00406948 00000000 fee1dead 28121969 01234567 73299b00
[  165.396602] 5fc0: 00406948 00000000 00000000 00000058 be91abc8 00000000 be91ab60 004056f8
[  165.404781] 5fe0: 00000058 be91aabc b6ed4d45 b6e56746

Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Signed-off-by: Lee Jones <lee.jones@linaro.org>