[ARM] 5399/1: [AT91] Configure MCLK and SSC for AT91SAMG20-EK

The AT91SAM20-EK has a WM8731 attached to it with MCLK supplied from
PCLK0 and the digital audio interface supplied by SSC0.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Andrew Victor <linux@maxim.org.za>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>