arm64: KVM: Avoid marking pages as XN in Stage-2 if CTR_EL0.DIC is set

On systems where CTR_EL0.DIC is set, we don't need to perform
icache invalidation to guarantee that we'll fetch the right
instruction stream.

This also means that taking a permission fault to invalidate the
icache is an unnecessary overhead.

On such systems, we can safely leave the page as being executable.

Acked-by: Catalin Marinas <catalin.marinas@arm.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Christoffer Dall <christoffer.dall@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>