tlv320adc3xxx: Allow MICBIAS pins to be used as

tlv320adc3xxx: Allow MICBIAS pins to be used as

Merge series from Ricard Wanderlof <ricard.wanderlof@axis.com>:

In some cases, depending on system design, the MICBIAS pins on the
chip are not needed as such, but a couple of extra GPIO pins would be
useful. This patch allows the MICBIAS pins to be configured in the
device tree as general purpose output pins, controlled via the GPIO
framework.

Owing to their originally intended purpose there are some limitations:
when the MICBIAS pins are deactivated, they will float, so will likely
need a pulldown in many applications. When activated, they will
assume the voltage specified by the micbias1-vg and micbias2-vg
properties, respectively, meaning that the resulting output voltage
will be 2.0 V, 2.5 V or AVDD .