mfd: pm8xxx-spk: Add support for class D speaker amplifier driver

The driver adds support for configuring the following parameters for
external pmic speaker amp driver
1. Gain
2. Mute/Unmute
3. Speaker enable/Disable
The above operations are supported by driver by exported apis
from kernel space.The Machine driver from ALSA would use these
to configure speaker.

Change-Id: I9817f5d5c2952ca423b84f35162a842123e4d413
Signed-off-by: Asish Bhattacharya <asishb@codeaurora.org>