mtd: rawnand: Add a helper to clarify the interface configuration

Name it nand_interface_is_sdr() which will make even more sense when
nand_interface_is_nvddr() will be introduced.

Use it when relevant.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20210505213750.257417-2-miquel.raynal@bootlin.com