CPU does not received signals for interrupts with a priority masked by
ICC_PMR_EL1. This means the CPU might not come back from a WFI
instruction.

Make sure ICC_PMR_EL1 does not mask interrupts when doing a WFI.

Since the logic of cpu_do_idle is becoming a bit more complex than just
two instructions, lets turn it from ASM to C.
Signed-off-by: NJulien Thierry <julien.thierry@arm.com>
Suggested-by: NDaniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: NCatalin Marinas <catalin.marinas@arm.com>
Reviewed-by: NMarc Zyngier <marc.zyngier@arm.com>
Cc: Will Deacon <will.deacon@arm.com>
Signed-off-by: NCatalin Marinas <catalin.marinas@arm.com>