xen: arm: rework placement of fdt in initial dom0 memory map

xen: arm: rework placement of fdt in initial dom0 memory map

The 32-bit Linux kernel uses its lowmem direct mapping to access the FDT. The
lowmem mapping is around 0.75GiB but varies depending on the kernel's .config.
Our current scheme of loading the FDT as high as 4GB therefore fails with
larger amounts of dom0 RAM.

The upstream documentation has recently been update to provide more guidance
<http://www.arm.linux.org.uk/developer/patches/viewpatch.php?id=7824/1>. In
accordance with this load the kernel just below 128MiB (aligned to 2MB) and
the FDT just above, or if there is less RAM available then as high as
possible.

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