arm64: unify smp_psci.c and psci.c

The functions in psci.c are only used from smp_psci.c, and smp_psci
cannot function without psci.c. Additionally psci.c is built when !SMP,
where it's expected that cpu_suspend may be useful.

This patch unifies the two files, removing pointless duplication and
paving the way for PSCI support in UP systems.

Signed-off-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>