drivers/gpio: stm32: Review code for Port G clock on L4/L5

On L4/L5 device, GPIO port G benefits from a dedicated supply
rail that should be enabled independently.
Review the code around this:
-Compile only when port G is enabled
-Assume that PWR clock is ON, as it is enabled as part of clock init

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>