dts: stm32-pinctrl.yaml: remove calculation of pinmux

- remove calculation of pinmux value for stm32 in device tree binding
  documentation.
- stm32-pinctrl.yaml contains wrong calculation as can be compared to
  #define STM32_PINMUX in stm32-pinctrl.h.
- stm32f1-pinctrl.yaml also contains different wrong calculation
  compared to #define STM32F1_PINMUX in stm32f1-pinctrl.h.

Signed-off-by: Andreas Klinger <ak@it-klinger.de>