mfd: twl6040: Add initial support

TWL6040 IC provides analog high-end audio codec functions for
handset applications. It contains several audio analog inputs
and outputs as well as vibrator support. It's connected to the
host processor via PDM interface for audio data communication.
The audio modules are controlled by internal registers that
can be accessed by I2C and PDM interface.

TWL6040 MFD will be registered as a child of TWL-CORE, and will
have two children of its own: twl6040-codec and twl6040-vibra.

This driver is based on TWL4030 and WM8350 MFD drivers.

Signed-off-by: Misael Lopez Cruz <misael.lopez@ti.com>
Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>