xen: arm: reduce the size of the xen heap to max 1/8 RAM size

xen: arm: reduce the size of the xen heap to max 1/8 RAM size

When building a 1GB dom0 on a system with 2GB RAM we are running out of domheap
pages, while there are still plenty of xenheap pages spare.

I would have sworn that when the domheap was exhausted we would fall back to
allocating xenheap pages but this doesn't appear to be the case. It's possible
that we have setup something incorrectly on ARM but alloc_domheap_pages pretty
clearly tries to allocate memory from MEMZONE_XEN+1..zone_hi.

Without the fallback from domheap to xenheap taking 1GB of any system with >1GB
of RAM for xenheap is excessive so instead set a limit of 1/8 of the total
amount of RAM. By way of comparison x86_32 used to have a static 12MB xenheap
(which also included .text etc) and in theory supported up to 16GB RAM, by that
measure 1/8 is plenty.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Tim Deegan <tim@xen.org>