ASoC: pxa: TTC DKB audio needs I2C

The missing dependency can lead to build errors, so
make it explicit in Kconfig.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Xia Kaixu <kaixu.xia@linaro.org>
Signed-off-by: Mark Brown <broonie@linaro.org>