Xen: ARM: Add support for mapping platform device mmio

Add a bus_notifier for platform bus device in order to map the device
mmio regions when DOM0 booting with ACPI.

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