mtd: rawnand: Add 'depends on HAS_IOMEM' where missing

When COMPILE_TEST is allowed and the platform needs uses the iomem API
we need to add an explicit dependency on HAS_IOMEM to avoid selection
of these drivers when building for an arch that has no iomem support
(this is the case of arch/um).

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>