Xen: Support adding DT nodes

Support adding xen,xen-bootargs node via --with-xen-cmdline to the
configure script and automatically add the Dom0 node to the DT as well.

Signed-off-by: Christoffer Dall <christoffer.dall@linaro.org>
Signed-off-by: Andre Przywara <andre.przywara@arm.com>
Tested-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
[Mark: s/XEN_BOOTARGS/XEN_CHOSEN/]
Signed-off-by: Mark Rutland <mark.rutland@arm.com>