dts: gpio: create gpio dt-bingings and inlude in stm32 dtsi files

Create a dt-bindings/gpio.h file.
Bindings definitions are extracted from existing gpio.h.
gpio dt-bindings file is required because existing gpio.h file could
not be parsed by dts parser.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>