mtd: rawnand: atmel: add generic name for EBICSA 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 EBICSA regmap, as a prerequisite for
sam9x60 nand controller support.

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