ASoC: wcd-mbhc: Add support for hph auto pull down

Pull down switches on headphone outputs need to be
enabled after accessory detection is completed to
eliminate noise on headphone when no audio gets
played and if device is getting charged through
usb cable. Add support to enable headphone auto
pull down.

Change-Id: Ife83a5843782e8b0e8238be6eaebbeccec4e929d
Signed-off-by: Phani Kumar Uppalapati <phaniu@codeaurora.org>