ARM: Xen: Document UEFI support on Xen ARM virtual platforms

Add a "uefi" node under /hypervisor node in FDT, then Linux kernel could
scan this to get the UEFI information.

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