ARM: dts: stm32: change pinctrl bindings definition

Initially each pin was declared in "include/dt-bindings/stm32<SOC>-pinfunc.h"
and each definition contained SOC names (ex: STM32F429_PA9_FUNC_USART1_TX).
Since this approach was approved, the number of supported MCU has
increased (STM32F429/STM32F469/STM32f746/STM32H743). To avoid to add a new
file in "include/dt-bindings" each time a new STM32 SOC arrives I propose
a new approach which consist to use a macro to define pin muxing in device
tree. All STM32 will use the common macro to define pinmux. Furthermore, it
will make STM32 maintenance and integration of new SOC easier .

Signed-off-by: Alexandre TORGUE <alexandre.torgue@st.com>
Reviewed-by: Vikas MANOCHA <vikas.manocha@st.com>
Reviewed-by: Benjamin Gaignard <benjamin.gaignard@linaro.org>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>