ALSA: ASoC: Make TLV320AIC26 user-visible

The TLV320AIC26 Kconfig option is unusual in that it supports the
OpenFirmware machine driver which doesn't have a hard binding to the
codec driver but discovers the codec via the device tree.  This makes it
meaningful to select the codec without a machine driver.

Ideally there would be a proxy entry so that this option was only
visible on OpenFirmware systems.

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