We plan to use vgic_find_mmio_region in vgic-its.c so let's
turn it into a public function.

Also let's take the opportunity to rename the region parameter
into regions to emphasize this latter is an array of regions.
Signed-off-by: NEric Auger <eric.auger@redhat.com>
Reviewed-by: NAndre Przywara <andre.przywara@arm.com>
Acked-by: NMarc Zyngier <marc.zyngier@arm.com>
Acked-by: NChristoffer Dall <cdall@linaro.org>