Use available DM stm32f7_gpio.c and pinctrl_stm32.c drivers
instead of board GPIO initialization.
Remove stm32_gpio.c which is no more used and migrate
structs stm32_gpio_regs and stm32_gpio_priv into
arch-stm32f4/gpio.h to not break compilation.
Signed-off-by: Patrice Chotard <patrice.chotard@st.com>