[ARM] generic-v8: Fix GIC compatible string

We should be using "arm,cortex-a15-gic" as GIC compatible string
for Generic-v8 board instead of "arm,cortex-a9-gic" because GIC
with virtualization extension is only available for Cortex-A15
GIC or later GIC versions.

Signed-off-by: Anup Patel <anup@brainfault.org>