ASoC: simple-amplifier: Make gpio property optional

Some amplifier may not have a GPIO to control the power, but instead simply
rely on the regulator to power up and down the amplifier.

In order to support those setups, let's make the GPIO optional.

Signed-off-by: Mylène Josserand <mylene.josserand@bootlin.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Signed-off-by: Mark Brown <broonie@kernel.org>