Enable building the driver as a loadable kernel module.

Link: https://lore.kernel.org/r/20210531085934.2662457-3-luca@lucaceresoli.netSigned-off-by: NLuca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: NLorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: NKishon Vijay Abraham I <kishon@ti.com>