arm: stm32: cleanup arch gpio.h

Cosmetic update of gpio.h:
- remove enumerate: stm32_gpio_port, stm32_gpio_pin
  because STM32_GPIO_XXX values are unused
- move STM32_GPIOS_PER_BANK in stm32_gpio.c
  as its value is IP dependent and not arch dependent

No functional change as number of banks and number of gpio by banks
is managed by device tree since since DM migration and
commit 8f651ca6 ("pinctrl: stm32: Add get_pins_count() ops").

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-by: Patrice Chotard <patrice.chotard@st.com>