arm: kernel: refactor the CPU suspend API for retention states

CPU suspend is the standard kernel interface to be used to enter
low-power states on ARM32 systems. Current cpu_suspend implementation
by default assumes that all low power states are losing the CPU context,
so the CPU registers must be saved and cleaned to DRAM upon state
entry. Furthermore, the current cpu_suspend() implementation assumes
that if the CPU suspend back-end method returns when called, this has
to be considered an error regardless of the return code (which can be
successful) since the CPU was not expected to return from a code path
that is different from cpu_resume code path - eg returning from the reset
vector.

All in all this means that the current API does not cope well with
low-power states that preserve the CPU context when entered
(ie retention states), since first of all the context is saved for nothing
on state entry for those states and a successful state entry can return as
a normal function return, which is considered an error by the current CPU
suspend implementation.

This patch refactors the cpu_suspend() API so that it can be split in
two separate functionalities. The arm32 cpu_suspend API just provides
a wrapper around CPU suspend operation hook. A new function is
introduced (for architecture code use only) for states that require
context saving upon entry:

__cpu_suspend(unsigned long arg, int (*fn)(unsigned long))

__cpu_suspend() saves the context on function entry and calls the
so called suspend finisher (ie fn) to complete the suspend operation.
The finisher is not expected to return, unless it fails in which case
the error is propagated back to the __cpu_suspend caller.

The API refactoring results in the following pseudo code call sequence
for a suspending CPU, when triggered from a kernel subsystem:

/*
 * int cpu_suspend(unsigned long idx)
 * @idx: idle state index
 */
{
-> cpu_suspend(idx)
	|---> CPU operations suspend hook called, if present
		|--> if (retention_state)
			|--> direct suspend back-end call (eg PSCI suspend)
		     else
			|--> __cpu_suspend(idx, &back_end_finisher);
}

By refactoring the cpu_suspend API this way, the CPU operations back-end
has a chance to detect whether idle states require state saving or not and
can call the required suspend operations accordingly either through simple
function call or indirectly through __cpu_suspend() which carries out
state saving and suspend finisher dispatching to complete idle state entry.

Change-Id: I02567729f882fb7c8e1f9ba054c55aeb8be4c31c
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>