drivers: flash: flash_stm32g4x.c: manage bank1/2 discontinuity
drivers: flash: flash_stm32g4x.c: manage bank1/2 discontinuity

When flash is Dualbank and flash size is lower than 512K,
then there is a discontinuity between bank1 and bank2.

Signed-off-by: Alexandre Bourdiol <c23cc711c078d3e6bb50b3dc45a1b8842691e59a@st.com>

galak/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,Vudentz/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,Vudentz/zephyr,galak/zephyr,Vudentz/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr,nashif/zephyr,nashif/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,finikorg/zephyr,galak/zephyr,Vudentz/zephyr