ArmVirtPkg/ArmVirtXen: add ARM support

ArmVirtPkg/ArmVirtXen: add ARM support

This adds ARM support to the ArmVirtXen platform. As is the case for
AARCH64, the ARM port adheres to the ARM Linux boot protocol, i.e.,
it expects the address of a DTB describing the platform to be passed
in r2, and relocates itself at runtime to the actual load time memory
offset.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Acked-by: Laszlo Ersek <lersek@redhat.com>
git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@19333 6f19259b-4bc3-4df7-8a09-765794883524