arm/efi: Use dom0less configuration when using EFI boot

arm/efi: Use dom0less configuration when using EFI boot

This patch introduces the support for dom0less configuration
when using UEFI boot on ARM, it permits the EFI boot to
continue if no dom0 kernel is specified but at least one domU
is found.

Introduce the new property "xen,uefi-binary" for device tree boot
module nodes that are subnode of "xen,domain" compatible nodes.
The property holds a string containing the file name of the
binary that shall be loaded by the uefi loader from the filesystem.

Introduce a new call efi_check_dt_boot(...) called during EFI boot
that checks for module to be loaded using device tree.
Architectures that don't support device tree don't have to
provide this function.

Update efi documentation about how to start a dom0less
setup using UEFI

Signed-off-by: Luca Fancellu <luca.fancellu@arm.com>
[stefano: drop inline from efi_check_dt_boot]
Signed-off-by: Stefano Stabellini <stefano.stabellini@xilinx.com>
Reviewed-by: Bertrand Marquis <bertrand.marquis@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Acked-by: Jan Beulich <jbeulich@suse.com>