ANDROID: gic-v3: Export gic_resume() for vendor GIC enhancements

syscore ops in gic-v3 takes care of invoking gic_resume() when
exiting from "deep" suspend. However for "s2idle" suspend syscore
ops will not get invoked.

Vendor modules can register for s2idle notifications and
invoke gic_resume() when the first cpu is waking up from s2idle.

Bug: 190353898
Change-Id: I3d565ed3fd0fcada2231b10415dcf487fefc7c5f
Signed-off-by: Maulik Shah <mkshah@codeaurora.org>