ARM: dts: at91: sama5d2: add ssc0 definition

The sama5d2 SoC has Synchronous Serial Controller which provides
synchronous communication link with external devices.
It's generally used in audio and telecom applications such as
I2S, Short Frame Sync, Long Frame Sync.

Signed-off-by: Alex Gershgorin <alex.gershgorin@qcore.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>