ASoC: omap: rx51: Enable McBSP2 sidetone

McBSP sidetone is needed in telephony applications. McBSP sidetone is a
configurable FIR filter that forms a loopback from McBSP input to output.
This patch enables the McBSP2 sidetone ALSA controls so that it can be used
on Nokia RX-51/N900.

Sidetone feature can be tested with following commands:

	(set up codec input and output paths)
	# Enable and configure sidetone
	amixer -D hw:0 set 'McBSP2 Sidetone' on
	amixer set -D hw:0 'McBSP2 Sidetone Channel 0' 32767
	echo 32767 >/sys/devices/platform/omap-mcbsp.2/st_taps
	# Do not loop audio via CPU
	arecord -f dat >/dev/null |aplay /dev/zero

Signed-off-by: Jarkko Nikula <jhnikula@gmail.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>