tests: drivers: flash on stm32l475 requires higher stack size
tests: drivers: flash on stm32l475 requires higher stack size

Increasing the stack size to pass the
tests/drivers/flash testcase on the disco_l475_iot1 disco kit.
Note that CONFIG_MPU_ALLOW_FLASH_WRITE is set.

Signed-off-by: Francois Ramu <b83adc7f8a23ee911a37fcb61c4033581aae0b7a@st.com>

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