ARM: parse separate DT properties for different commandlines

ARM: parse separate DT properties for different commandlines

Currently we use the chosen/bootargs property as the Xen commandline
and rely on xen,dom0-bootargs for Dom0. However this brings issues
with bootloaders, which usually build bootargs by bootscripts for a
Linux kernel - and not for the entirely different Xen hypervisor.

Introduce a new possible device tree property "xen,xen-bootargs"
explicitly for the Xen hypervisor and make the selection of which to
use more fine grained:
- If xen,xen-bootargs is present, it will be used for Xen.
- If xen,dom0-bootargs is present, it will be used for Dom0.
- If xen,xen-bootargs is _not_ present, but xen,dom0-bootargs is,
  bootargs will be used for Xen. Like the current situation.
- If no Xen specific properties are present, bootargs is for Dom0.
- If xen,xen-bootargs is present, but xen,dom0-bootargs is missing,
  bootargs will be used for Dom0.

The aim is to allow common bootscripts to boot both Xen and native
Linux with the same device tree blob. If needed, one could hard-code
the Xen commandline into the DTB, leaving bootargs for Dom0 to be set
by the (non Xen-aware) bootloader.

I will send out a appropriate u-boot patch, which writes the content
of the "xen_bootargs" environment variable into the xen,xen-bootargs
dtb property.

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