ASoC: Use CODEC as clock master on AT91SAM9G20-EK

This simplifies the driver by removing the need to manually
configure dividers within the CPU and improve audio performance
by ensuring that the optimal phase relationships between the
clocks in the system are maintained.

Note that currently this means that for playback to work the
Output Mixer HiFi switch must be enabled since otherwise CODEC
will not generate the DAC clock.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>