ARM: dts: at91: sama5d2_xplained: add pin muxing and enable classd

This patch adds the pin muxing for classd and enables it.

Signed-off-by: Cyrille Pitchen <cyrille.pitchen@atmel.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Quentin Schulz <quentin.schulz@free-electrons.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>