arch: arm: stm32: f3/f4/l4: write output speed to hardware register

The ouput speed of the gpio pins passed via the 'conf' argument was
ignored, causing the speed to always be in its reset state (lowest
possible speed for most pins). This was causing problems for pins that
actually need a speed faster than the default, like the ethernet
controller pins.

Combined with the correct pinmux configuration this fixes problems
of the olimex_stm32_e407 board not being able to send ethernet data.

Signed-off-by: Erwin Rol <erwin@erwinrol.com>