ASoC: pxa: make TTC DKB tristate

In a rare combination of Kconfig settings, the 88pm860x-codec
module may be selected as a loadable module, while it's also being
used by the ttb-dkb code that is built-in, resulting in a link
error:

sound/built-in.o: In function `ttc_pm860x_init':
:(.text+0x3e888): undefined reference to `pm860x_hs_jack_detect'
:(.text+0x3e898): undefined reference to `pm860x_mic_jack_detect'

Changing ttb-tkb to a tristate option tells Kconfig that 88pm86x
actually needs to be built-in if ttc-dkb is also built-in.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Mark Brown <broonie@kernel.org>