samples/drivers/flash_shell: Add nucleo_f746zg to platform_whitelist
samples/drivers/flash_shell: Add nucleo_f746zg to platform_whitelist

In order to enable build of stm32f7 flash driver in sanitytests,
add nucleo_f746zg to flash_shell platform whitelist.

Signed-off-by: Erwan Gouriou <de4d967452b83bb0e37525652734e7d9163b6a4c@linaro.org>

Vudentz/zephyr,finikorg/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,nashif/zephyr,zephyrproject-rtos/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,explora26/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,finikorg/zephyr,nashif/zephyr,explora26/zephyr,Vudentz/zephyr,nashif/zephyr,finikorg/zephyr,Vudentz/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,galak/zephyr,ldts/zephyr,galak/zephyr,galak/zephyr,nashif/zephyr,finikorg/zephyr,explora26/zephyr,ldts/zephyr,ldts/zephyr,explora26/zephyr,GiulianoFranchetto/zephyr