ARM: dts: at91: sama5d2 Xplained: add pin muxing for I2S

This patch sets the pin muxing for the I2S controllers

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
[codrin.ciubotariu@microchip.com: added pin muxing for the second
controller]
Signed-off-by: Codrin Ciubotariu <codrin.ciubotariu@microchip.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>