mux: gpio: add optional regulator support

Some of the external muxes needs powering up using a regulator.
This is the case with Lenovo T14s laptop which has a external audio mux
to handle US/EURO headsets.

Add support to the driver to handle this optional regulator.

Signed-off-by: Srinivas Kandagatla <srinivas.kandagatla@linaro.org>
Tested-by: Christopher Obbard <christopher.obbard@linaro.org>
Reviewed-by: Johan Hovold <johan+linaro@kernel.org>
Tested-by: Johan Hovold <johan+linaro@kernel.org>
Link: https://lore.kernel.org/r/20250327100633.11530-3-srinivas.kandagatla@linaro.org


[krzk: Adjust dev_err message per Johan's review]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>