irqchip/gic: Restore CPU interface checking

When introducing the whole CPU feature detection framework,
we lost the capability to detect a mismatched GIC configuration
(using the GICv2 MMIO interface, but having the system register
interface enabled).

In order to solve this, use the new this_cpu_has_cap() helper.
Also move the check to the CPU interface path in order to catch
systems where the first CPU has been correctly configured,
but the secondaries are not.

Reviewed-by: Catalin Marinas <catalin.marinas@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>