drivers: firmware: psci: move power_state handling to generic code

Functions implemented on arm64 to check if a power_state parameter
is valid and if the power_state implies context loss are not
arm64 specific and should be moved to generic code so that they
can be reused on arm systems too.

This patch moves the functions handling the power_state parameter
to generic PSCI firmware layer code.

Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Acked-by: Sudeep Holla <sudeep.holla@arm.com>
Tested-by: Jisheng Zhang <jszhang@marvell.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>