memory: atmel-ebi: Simplify SMC config code

New helpers/macros have been to atmel-smc.h introduced to simplify SMC
regs manipulation. Rework the code to use those helpers, and simplify
the ->xlate_config(), ->get_config() and ->apply_config() implementations.

SMC configs are now stored in a struct atmel_smc_cs_conf object that
directly contains registers values, which should help implementing
->suspend()/->resume() hooks.

We can also get rid of those regmap fields (and the associated ->init()
hook) which are not longer needed thanks to the
atmel_[h]smc_cs_conf_{apply,get}() helpers.

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>