[ARM] Add common psci.h for sharing between smp_psci.c and emulate_psci.c

The PSCI host (i.e. smp_psci.c) and PSCI emulation (i.e. emulate_psci.c)
should use the same defines of PSCI for consistency hence this patch
introduce common psci.h for this purpose.

Signed-off-by: Anup Patel <anup@brainfault.org>