samples: flash_shell: Run on boards without zephyr,flash-controller

The flash shell can now build when there isn't a chosen
zephyr,flash-controller available, so we can simplify the filter to
apply this sample to more boards.

Two STM32 H7 board configurations are excluded because the flash driver
isn't supported yet on the M4 core.

Signed-off-by: Maureen Helm <maureen.helm@nxp.com>