mfd: syscon: Add Atmel SMC binding doc

The SMC registers are used to configure Atmel EBI (External Bus Interface)
to interface with standard memory devices (NAND, NOR, SRAM or specialized
devices like FPGAs).

Declare this memory region as a syscon, so that different drivers can
configure the SMC interface (mostly timing configuration) according to
their need.

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>