ARM: dts: at91: Map MCLK for wm8731 on at91sam9g20ek

The MCLK of the WM8731 on the AT91SAM9G20-EK board is connected to the
PCK0 output of the SoC and is expected to be set to 12MHz. Previously
this was mapped using pre-common clock API calls in the audio machine
driver but the conversion to the common clock framework broke that so
describe things in the DT instead.

Fixes: ff78a189

 ("ARM: at91: remove old at91-specific clock driver")
Signed-off-by: Mark Brown <broonie@kernel.org>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220404102806.581374-2-broonie@kernel.org