memory: atmel-ebi: Stop using reg_field objects for simple things

Turn the ->ebi_csa reg field into a simple offset that can be used with
with the matrix regmap. Using reg fields was overkill for this use case.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>