ASoC: rt5670: add runtime PM support.

This patch adds runtime PM support.

Mendong update:
- call  pm_runtime_set_active() to set intial power status to active, to
  be consist with its intial ACPI power state D0. The codec will be suspended
  after probe and runtime status change will trigger ACPI D3 method to turn
  off MCLK for power saving.
- switch to internal clock when runtime suspended, since BIOS turn off MCLK
  in D3.