xen/arm: Correctly boot with an initrd and no linux command line

xen/arm: Correctly boot with an initrd and no linux command line

When DOM0 device tree is building, the properties for initrd will
only be added if there is a linux command line. This will result to a panic
later:

(XEN) *** LOADING DOMAIN 0 ***
(XEN) Populate P2M 0x20000000->0x40000000 (1:1 mapping for dom0)
(XEN) Loading kernel from boot module 2
(XEN) Loading zImage from 0000000001000000 to 0000000027c00000-0000000027eafb48
(XEN) Loading dom0 initrd from 0000000002000000 to 0x0000000028200000-0x0000000028c00000
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) Cannot fix up "linux,initrd-start" property
(XEN) ****************************************
(XEN)

Signed-off-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>