platform/mellanox: mlxreg-hotplug: Enable building for ARM

Add ARM in addition to X86 as supported architectures in the Mellanox
Hotplug Platform driver Kconfig entry.

Signed-off-by: Vadim Pasternak <vadimp@mellanox.com>
[dvhart: rewrite commit message]
Signed-off-by: Darren Hart (VMware) <dvhart@infradead.org>