arm, arm64: KVM: handle potential incoherency of readonly memslots

Readonly memslots are often used to implement emulation of ROMs and
NOR flashes, in which case the guest may legally map these regions as
uncached.
To deal with the incoherency associated with uncached guest mappings,
treat all readonly memslots as incoherent, and ensure that pages that
belong to regions tagged as such are flushed to DRAM before being passed
to the guest.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>