ARM: dts: at91: sama5d2: add SFR node

This SFR node is looked up by the I2S controller driver to tune the
SFR_I2SCLKSEL register.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Ludovic Desroches <ludovic.desroches@atmel.com>
Acked-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>