[ALSA] soc - Add Invert Switch for ROUT2

GTA02 device has a speaker between LOUT2 & ROUT2 and in this mode ROUT2
needs to be inverted. This patch adds a mixer control for this.

Signed-off-by: Graeme Gregory <graeme@openmoko.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>