ASoC: mchp-spdiftx: add power saving features

Merge series from Claudiu Beznea <claudiu.beznea@microchip.com>:

This series adds support for runtime PM and system suspend/resume
for Microchip SPDIFTX (patches 2/3, 3/3). Along with it I took the
chance and added a minor cleanup (patch 1/3).