ASoC: Disable microphone input for AT91SAM9G20-EK by default

As shipped the board does not have inputs but it is relatively
straightforward to modify the board to hook them up so support
is provided in the driver. When these modifications have not
been made enabling the microphone stage can cause problems.

Add an ifdef to disable this by default. Don't put it into
Kconfig since users will have to get their soldering irons
out to change things.

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