mfd: syscon: Update Atmel SMC binding doc

A new compatible string is introduced for SMC on sama5d2 to manage a
different layout of the registers.

Signed-off-by: Ludovic Desroches <ludovic.desroches@microchip.com>
│Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>