gpio: dts: Introduce Kconfig symbols to convey GPIO dts support

Add a set of Kconfig symbols that allow us to set that the GPIO
driver/SoC support DTS (HAS_DTS_GPIO) and that drivers that need/use
GPIO support DTS as well (HAS_DTS_GPIO_DEVICE).

Signed-off-by: Kumar Gala <kumar.gala@linaro.org>