drivers: flash: stm32: add a weak flash_stm32_valid_range()

Most implementations have the same logic, with only a different write
block size. Now that we are using write-block-size from the device tree,
it is possible to use a default implementation that can be overridden if
necessary.

Signed-off-by: Florian Vaussard <florian.vaussard@gmail.com>