dt-bindings: pinctrl: stm32: fix STM32*_PINMUX docs location

Documentation for STM32*_PINMUX macros was incorrectly placed above the
macro, so it was in practice documenting the next macro (e.g.
STM32_MODE_SHIFT).

Signed-off-by: Gerard Marull-Paretas <gerard.marull@nordicsemi.no>