drivers: flash: Build STM32 flash support only when selected

The current configuration causes the STM32 flash support always to be
built, even if an unrelated flash driver, by example the SPI_NOR driver
is selected.
This behaviour gets especially problematic (build failure) if the flash
hardware of the given MCU is not supported (e.g. STM32F2).
The suggested change should ensure that STM32 flash support only is
built when it actually is selected.

Signed-off-by: Marco Peter <marco@peter-net.ch>