xen/arm: Allocate memory for dom0 from the bottom with the 1:1 Workaround

xen/arm: Allocate memory for dom0 from the bottom with the 1:1 Workaround

On Linux, the option CONFIG_ARM_PATCH_PHYS_VIRT (by default enabled) allows
the Kernel to be loaded anywhere (or nearly) by patching the translation
pv<->virt at boot time.

The current solution in Linux assuming that the delta physical address -
virtual address is always negative. A positive delta will destroy all the
optimisation to modify only a part of the translation instruction (add/sub).

By default, Xen is allocating memory from the top of memory and then
goes down. To avoid booting issue with Linux, we must allocate memory
from the bottom (ie starting from 0).

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