ASoC: rt5677: Add a configuration option for LDO2_POW pin

Some boards have this pin statically tied and do not require any configuration,
some other boards allow to enable chip using GPIO.

Add an option that tells which GPIO is used to power the audio codec.

Signed-off-by: Anatol Pomozov <anatol.pomozov@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>