xen/arm: use !CONFIG_NUMA to keep fake NUMA API

xen/arm: use !CONFIG_NUMA to keep fake NUMA API

We have introduced CONFIG_NUMA in a previous patch. And this
option is enabled only on x86 at the current stage. In a follow
up patch, we will enable this option for Arm. But we still
want users to be able to disable the CONFIG_NUMA via Kconfig. In
this case, keep the fake NUMA API, will make Arm code still
able to work with NUMA aware memory allocation and scheduler.

Signed-off-by: Wei Chen <wei.chen@arm.com>
Tested-by: Jiamei Xie <jiamei.xie@arm.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>