ASoC: twl6040: Add ramp up/down volume for HS and HF

Add ramp functions for the headset and handsfree outputs
in order to reduce the pops during power on/off sequences.

In order to give more control to volume ramp, step size and delay
between steps can be specified.

The patches are based on wm8350 implementation from Liam
Girdwood.
Signed-off-by: Margarita Olaya Cabrera <magi.olaya@ti.com>
Signed-off-by: Misael Lopez Cruz <misael.lopez@ti.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Liam Girdwood <lrg@slimlogic.co.uk>