mtd: rawnand: sunxi: Allow selection of this driver when COMPILE_TEST=y

It just makes NAND maintainers' life easier by allowing them to
compile-test this driver without having ARCH_SUNXI 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>