ASoC: SPEAr spdif_out: Add spdif out support

This patch implements the spdif out driver for ST peripheral. This
peripheral implements IEC60958 standard

Signed-off-by: Vipin Kumar <vipin.kumar@st.com>
Signed-off-by: Rajeev Kumar <rajeev-dlh.kumar@st.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>