soc: stm32: fix FLASH_BASE_ADDR for apps linked in ext Q/OSPI Flash

Following changes in e35ac8f9 and 14c1b4aa to how external Q/OSPI Flash
nodes are declared in DT for boards with STM32 SoCs, FLASH_BASE_ADDRESS
needs to be set manually similar to XSPI Flash, so that it does not
default to (dt_chosen_reg_addr_hex,$(DT_CHOSEN_Z_FLASH)) which gives 0.
This change is critical for running apps with MCUboot from external
Q/OSPI Flash.

Signed-off-by: Abderrahmane JARMOUNI <git@jarmouni.me>