xen: arm: support bootmodule type detection by ordering

xen: arm: support bootmodule type detection by ordering

Assign modules types based on the order in which they are defined in the FDT.
This is supported only for the dom0 kernel and ramdisk when given as the first
and second modules respectively, similar to how
http://wiki.xen.org/wiki?title=Xen_ARM_with_Virtualization_Extensions/Multiboot&oldid=11824
defined the default types from the bootloader side.

This is compatible with how Xen interprets the modules with x86 multiboot and I
think simplifies things for bootloaders which now need not contain similar
guessing code if they only care about the most basic case.

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