KVM: arm/arm64: Export mmio_read/write_bus

Rename mmio_{read,write}_bus to kvm_mmio_{read,write}_bus and export
them out of mmio.c.
This will be needed later for the new VGIC implementation.

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