mfd: syscon: Add Atmel Matrix bus DT binding documentation

The Matrix registers are provided to configure internal bus behavior on
at91 SoCs.
Some registers might be accessed by several drivers (e.g. to configure
external memory bus timings), hence we declare this register set as a
syscon device.

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>