arm: psci: make psci usable on single core socs

PSCI can be used on both multiple and single core socs. Current
implementation only allows PSCI to work on multiple core socs.
This patch removes this restriction so that PSCI can work on
single core socs as well.

Signed-off-by: Chenhui Zhao <chenhui.zhao@nxp.com>
Signed-off-by: Tang Yuantian <andy.tang@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>