[ARM] 5596/1: at91sam9g20-ek: Register WM8731 in board file

The WM8731 driver has been updated to allow registration via normal
device model methods rather than from within the ASoC driver probe
so update the AT91SAM9G20-EK to make use of this.

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>