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.

Due to how the U-Boot driver model works (where a single device can only
implement a single drivers) we need to add a bunch of boilerplate here
where we have a nop device which binds to the DT node and handles
initialising common data, with two children (for the pinctrl and GPIO).

Signed-off-by: Caleb Connolly <caleb.connolly@linaro.org>