xen: arm: enable balloon driver

The code is now in a state where can just enable it.

Drop the *_xenballloned_pages duplicates since these are now supplied
by the balloon code.

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Acked-by: Stefano Stabellini <stefano.stabellini@eu.citrix.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>