ASoC: sti: helper functions for unip tdm slots configuration

- sti_uniperiph_set_tdm_slot: store tdm slot config in unip context
- sti_uniperiph_get_tdm_word_pos: configure unip tdm slots pos regs

Signed-off-by: Moise Gergaud <moise.gergaud@st.com>
Acked-by: Arnaud Pouliquen <arnaud.pouliquen@st.com>
Signed-off-by: Mark Brown <broonie@kernel.org>