ASoC: mmp-pcm: Staticize non exported structs and functions

The mmp_pcm_ops and mmp_soc_platform struct as well as the mmp_pcm_new()
function are only used in mmp-pcm.c, so make them static.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>