mtd: rawnand: gpmi: Introduce GPMI_IS_MXS() macro

Introduce a GPMI_IS_MXS() macro to take into account the cases
when mx23 or mx28 are used, which helps readability.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>