boot-wrapper: arm64: add support for GICv3

Performs the minimal initialization required for GICv3 support.
Support can be enabled with --enable-gicv3.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Mark Rutland <mark.rutland@arm.com>