xen: arm: fix boot on arndale.

xen: arm: fix boot on arndale.

The differences between Arndale and the Odoid-XU are more interesting
than first though, which results in 0bf8ddecb4df "xen/arm: Add
support for the Odroid-XU board." breaking boot on arndale.

Revert back to arndale compatible behaviour while we sort this out.
Specifically we must (counterintuitively) use the regular (!ns)
sysram and the correct offset is 0x0 and 0x1c.

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