soc: arm: st_stm32: Cleanup gpio function code.

All series STM32 have mostly the same GPIO architecture
and can share the same code for GPIO manipulation.
Functions of the external interrupt line control are also the same.
This patch extracts common code from them and put them into the 'common'
folder.

Functions of control GPIO of these series scattered in
soc/arm/st_stm32/stm32xx/ folders contain these functions:
stm32_gpio_flags_to_conf(), stm32_gpio_configure(), stm32_gpio_set(),
stm32_gpio_get, stm32_gpio_enable_int().
This patch merges them into the gpio_stm32.c file.

Signed-off-by: Song Qiang <songqiang1304521@gmail.com>