xen: arm: introduce uImage probe function for Dom0

xen: arm: introduce uImage probe function for Dom0

Patch adds a possibility to boot dom0 kernel from uImage.

uImage header format:
http://git.denx.de/cgi-bin/gitweb.cgi?p=u-boot.git;a=blob;f=include/image.h

Signed-off-by: Oleksandr Dmytryshyn <oleksandr.dmytryshyn@globallogic.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- fixed up coding style nit to be consistent with surrounding,
         but differently wrong, coding style.
         s/Not supported/Unsupported/
         s/uimage->arch/uimage.arch/ ]