ASoC: omap: rx51: Add FM transmitter support

Si4713 FM transmitter on Nokia RX-51/N900 is connected to same Line out
signals of TLV320AIC34 than TPA6130 headphone amplifier.

This patch adds route to transmitter and "FM Transmitter" control to keep
route active when needed.

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