xen: memory : Add new XENMAPSPACE type XENMAPSPACE_dev_mmio

Add a new type of Xen map space for Dom0 to map device's MMIO region.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Julien Grall <julien.grall@arm.com>
Tested-by: Julien Grall <julien.grall@arm.com>