arm-autonomy/xen-devictree: Update dom0_mem default inline with best practices

Updated default value for XEN_DEVICETREE_DOM0_MEM from "1024M" to
"1024M,max:1024" to match best practice for dom0_mem command line
argument. Documentation also updated to refer to new default value

Issue-Id: SCM-2037
Signed-off-by: Nathan Dunne <Nathan.Dunne@arm.com>
Change-Id: Ifa7e78bf22b024cb7fe4b782f628860ff44860ad
Signed-off-by: Jon Mason <jon.mason@arm.com>