dts: arm: stm32g0: add pwr node and wkup-pins

WKUP3 is only available for g0b0/1 and g0c1 variants.

WKUP5 is only available in larger packages. However, package sizes
are currently not considered in the devicetree file schema for STM32.

Signed-off-by: Martin Jäger <martin@libre.solar>