ASoC: ssm4567: Add support for disabling the boost stage

This patch adds a switch to enable/disable boost stage of the output
amplifier. Applications that know that they do not need the output
amplifier boost stage can disable it to conserve a bit of power.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Mark Brown <broonie@kernel.org>