dm: gpio: Add of-platdata support

Add support for accessing GPIOs using of-plata. This uses the same
mechanism as for clocks, but allows use of the xlate() method so that
the driver can interpret the parameters.

Update the condition for GPIO_HOG so that it is not built into SPL,
since it needs SPL_OF_REAL which is not enabled in sandbox_spl.

Signed-off-by: Simon Glass <sjg@chromium.org>