[RT5645] Add a sysfs-based override for hp-detect

- Adds a sysfs node `force_hp_detect` that accepts three values -- hp,
speaker, or plug. `hp` forces the codec to output on the headphone,
`speaker` forces the output on speakers, and `plug` uses the state of
the jack detect input.

Change-Id: I794670d5d547a630baa6b8752cb760eb51d76ae7