Revert "ARM: 8457/1: psci-smp is built only for SMP"

This reverts commit dbcfee72.

Upstream commit be95485 (ARM: 8457/1: psci-smp is built only for SMP)
was intended to fix the build error for configs with CONFIG_SMP=n and
CONFIG_ARM_PSCI=y, but it end up introducing a build error when
cherry-picked on 3.18.y.

This patch resulted in redefinition of psci_init() and broke the
build for every build config in 3.18.y with CONFIG_SMP=n and
CONFIG_ARM_PSCI=y.

Signed-off-by: Amit Pundir <amit.pundir@linaro.org>