dt-bindings: tas2770: Add shutdown gpio property

Add the shutdown-gpios property to the yaml to define the GPIO that can
be used to place the device in shutdown mode or wake the device up.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Link: https://lore.kernel.org/r/20200923132600.10652-1-dmurphy@ti.com


Signed-off-by: Mark Brown <broonie@kernel.org>