arm: KVM: Add optimized PIPT icache flushing

Calling __cpuc_coherent_user_range to invalidate the icache on
a PIPT icache machine has some pointless overhead, as it starts
by cleaning the dcache to the PoU, while we're guaranteed to
have already cleaned it to the PoC.

As KVM is the only user of such a feature, let's implement some
ad-hoc cache flushing in kvm_mmu.h. Should it become useful to
other subsystems, it can be moved to a more global location.

Reviewed-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>