imx: mx7: psci: improve cpu hotplug flow

imx: mx7: psci: improve cpu hotplug flow

This patch improves cpu hotplug, previous cpu_off
implementation is NOT safe, a CPU can NOT power down
itself in runtime, it will cause system bus hang due
to pending transaction. So need to use other online
CPU to kill it when it is ready for killed.

Here use SRC parameter register and a magic number
of ~0 as handshake for killing a offline CPU,
when the online CPU checks the psci_affinity_info,
it will help kill the offline CPU according to
the magic number stored in SRC parameter register.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>