GICv3: Allow either G1S or G0 interrupts to be configured

Currently the GICv3 driver mandates that platform populate
both G1S and G0 interrupts. However, it is possible that a
given platform is not interested in both the groups and
just needs to specify either one of them.

This patch modifies the `gicv3_rdistif_init()` & `gicv3_distif_init()`
functions to allow either G1S or G0 interrupts to be configured.

Fixes ARM-software/tf-issues#400

Change-Id: I43572b0e08ae30bed5af9334f25d35bf439b0d2b