soc: st: common: Rename STM32_PWR_WKUP_PIN_SRC_x

This renames the STM32_PWR_WKUP_PIN_SRC_x symbols to better match
their meaning. It also adds a new symbol (STM32_PWR_WKUP_PIN_NOT_MUXED)
for SoCs without wake-up mux support.

Signed-off-by: Tomáš Juřena <jurenatomas@gmail.com>