It just makes maintainers' life easier by allowing them to compile-test
this driver without having FSL_SOC, ARCH_LAYERSCAPE or SOC_LS1021A
enabled.

We also need to add a dependency on HAS_IOMEM to make sure the
driver compiles correctly.
Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>