xen: arm: rework dom0 initrd and dtb placement

xen: arm: rework dom0 initrd and dtb placement

This now uses the same decision tree as libxc (which is much easier to test).

The main change is to explicitly handle the placement at 128MB or end of RAM
as two cases, rather than combining with MIN. The effect is the same but the
code is clearer.

Secondly the attempt to place the modules right after the kernel is removed,
since it is redundant with the case where placing them at the end of RAM ends
up abutting the kernel.

Also round the kernel size up to a 2MB boundary.

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