irqchip/gic-v3-its: Make is_v4 use a TYPER copy

Instead of caching the GICv4 compatibility in a discrete way, cache the
TYPER register instead, which can then be used to implement the same
functionnality. This will get used more extensively in subsequent patches.

Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Zenghui Yu <yuzenghui@huawei.com>
Link: https://lore.kernel.org/r/20191027144234.8395-5-maz@kernel.org
Link: https://lore.kernel.org/r/20191108165805.3071-5-maz@kernel.org