rockchip: rk3399: Add bootcount support

rockchip: rk3399: Add bootcount support

Add bootcount support for Rockchip rk3399.

The bootcount value is preserved in PMU_SYS_REG0 register,
this would help to support redundent boot.

Once the redundant boot triggers, the altboot command
will look for extlinux-rollback.conf on particular
bootable partition which supposed to be a recovery
partition where redundant boot required.

Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>