ASoC: Add snd_soc_codec_{readable,writable}_register()

Provide the top level ASoC core functions for indicating whether
a given register is readable or writable.

Signed-off-by: Dimitris Papastamos <dp@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>