xen/arm: Fix crash if last memory section is bigger than 1gb

xen/arm: Fix crash if last memory section is bigger than 1gb

On arm32 the xenheap has a maximum size of 1GB. On systems with more than 8GB
(so 1/8 total RAM is greater than 1GB) there is no point in searching for a
region with 1/8 of the total RAM when only 1GB will be used. Therefore limit
the maximum size to 1GB before searching.

Signed-off-by: Frediano Ziglio <frediano.ziglio@huawei.com>
Reviewed-by: Julien Grall <julien.grall@linaro.org>
Acked-by: Ian Campbell <ian.campbell@citrix.com>
[ ijc -- updated commit message as discussed ]