memory: atmel-ebi: add generic name for ebi regmap

The sam9x60 board defines the CCFG_EBICSA register under SFR,
and not as a MATRIX register, as previous boards do. Add a
more generic name for the EBI regmap as a prerequisite for
sam9x60 ebi support.

Signed-off-by: Tudor Ambarus <tudor.ambarus@microchip.com>
Acked-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>