xen: balloon: enable for ARM

Since c275a57f "xen/balloon: Set balloon's initial state to number of
existing RAM pages" the balloon driver appears to work fine on ARM as far as I
can tell. Prior to that commit it was broken because on ARM RAM doesn't
typically start at zero, effectively leaving a big MMIO hole at the start.
This would cause the balloon driver to give away all of RAM at start of day,
which is rather inconvenient.

It was already enabled (or rather not excluded) on ARM64. The
c1d15f5c
"xen/balloon: Seperate the auto-translate logic properly (v2)"
added in the proper plumbing to work with ARM and PVH type guests.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Cc: David Vrabel <david.vrabel@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
[v2: Added the bit about PVH]
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>