arm: at91/spl: mpddrc: add struct atmel_mpddrc_config

Add struct atmel_mpddrc_config to accommodate the mpddrc register
configurations, not using the mpddrc register map structure,
struct atmel_mpddrc, in order to increase readability and reduce
run-time memory use.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Andreas Bießmann <andreas.devel@googlemail.com>