arm: armv7: introduce cpu suspend and resume support

Just like linux, it supports cpu save and restore context
during enter and exit low power mode. With this patch, cpu
is able to suspend with core

arm: armv7: introduce cpu suspend and resume support

Just like linux, it supports cpu save and restore context
during enter and exit low power mode. With this patch, cpu
is able to suspend with core power off.

Workflow for trap into ATF for system suspend:
cpu_suspend
-> cpu_do_suspend
-> arch specific fn: int (*fn)(unsigned long)
-> psci_system_suspend(deliver 'cpu_resume()' address to ATF)
-> ATF system suspend
<- ATF system resume
<- cpu_resume
<- cpu_do_resume
next instruction

Notice: If needed, you should remember to save and restore GIC by yourself.

Change-Id: I5cb6fb6ac5b6a7f4ec4a975b0fc38250b000b28e
Signed-off-by: Joseph Chen <chenjh@rock-chips.com>

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