arm/acpi: Permit MMIO access of Xen unused devices for Dom0

arm/acpi: Permit MMIO access of Xen unused devices for Dom0

Firstly it permits full MMIO capabilities for Dom0. Then deny MMIO
access of Xen used devices, such as UART, GIC, SMMU. Currently, it only
denies the MMIO access of UART and GIC regions. For other Xen used
devices it could be added later when they are supported.

Signed-off-by: Shannon Zhao <shannon.zhao@linaro.org>
Reviewed-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Acked-by: Julien Grall <julien.grall@arm.com>