arm: Fix compile warning for psci

Commit e71246a2 changes psci_init from a
function returning a void to an int, but does not change the non
CONFIG_ARM_PSCI implementation to return a value, which causes a compile
warning.  Just return 0.

Cc: Ashwin Chaugule <ashwin.chaugule@linaro.org>
Cc: Shawn Guo <shawn.guo@freescale.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>