mfd: syscon: Add atmel-smc registers definition

Atmel AT91 SoCs have a memory range reserved for SMC (Static Memory
Controller) configuration.
Expose those registers so that drivers can make use of the smc syscon
declared in at91 DTs.

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