gpio: qcom_pmic: add pinctrl driver

gpio: qcom_pmic: add pinctrl driver

Introduce a basic pinctrl driver for the SPMI PMIC GPIOs. This is
necessary to make proper use of upstream DT bindings specifically on the
dragonboard410c where they're used to switch between USB host and device
modes.

Only support for driving the pins as output low or high is enabled for
now.

To minimise duplicated code and allow for sharing common DT data, the
pinctrl driver is initialised as a child of the existing GPIO driver.

Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Reviewed-by: Sumit Garg <sumit.garg@linaro.org>
Tested-by: Sumit Garg <sumit.garg@linaro.org> #qcs404
Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>