PCI: dra7xx: Make it a kernel module

Enable building the driver as a loadable kernel module.

Link: https://lore.kernel.org/r/20210531085934.2662457-3-luca@lucaceresoli.net


Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Lorenzo Pieralisi <lorenzo.pieralisi@arm.com>
Acked-by: Kishon Vijay Abraham I <kishon@ti.com>