ASoC: omap: rx51: Add earphone support

Earphone in Nokia RX-51/N900 is connected to left HP output of B part of the
TLV320AIC34 dual codec. In RX-51 the codec A is used as a traditional codec
and the codec B as an auxiliary device.

Audio from codec A goes via the codec B to earphone:
MONO_LOUT of A -> LINE2R of B (B interconnects) -> HPLOUT of B -> Earphone.

Take earphone into use by utilizing the recent ASoC auxiliary and
cross-device support.

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>