arm/acpi: Create min DT stub for Dom0

arm/acpi: Create min DT stub for Dom0

Create a DT for Dom0 for ACPI-case only. DT contains minimal required
information such as Dom0 bootargs, initrd, efi description table and
address of uefi memory table.

Also document this device tree bindings of "hypervisor" and
"hypervisor/uefi" node.

Signed-off-by: Naresh Bhat <naresh.bhat@linaro.org>
Signed-off-by: Parth Dixit <parth.dixit@linaro.org>
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>