mfd: syscon: Add Atmel MC (Memory Controller) registers definition

The at91rm9200 SoC embeds a Memory Controller block which is used to
configure several aspects of the platform:
- AHB/APB Bus behavior
- SDRAM Controller
- EBI (External Bus Interface) and SMC (Static Memory Controller) config

Those registers might be accessed by different drivers, hence we need to
define it as a syscon device.

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