ARMv7: GICv2 driver can manage GICv1 with security extension

Some SoCs integrate a GIC in version 1 that is currently not supported
by the trusted firmware. This change hijacks GICv2 driver to handle the
GICv1 as GICv1 is compatible enough with GICv2 as far as the platform
does not attempt to play with virtualization support or some GICv2
specific power features.

Note that current trusted firmware does not use these GICv2 features
that are not available in GICv1 Security Extension.

Change-Id: Ic2cb3055f1319a83455571d6d918661da583f179
Signed-off-by: Etienne Carriere <etienne.carriere@linaro.org>