pinmux: stm32f4: Clean-up pinmux arrays

Clean-up the pinmux arrays as a preparatory work before adding more
pinmuxes.

This is achieved by the following two actions:
- Define the PAD macro to simplify the [x - 1] = y construct
- Reorder the declartions by bank / pin to make it easier to
  locate a pin among a high number of other pins, while minimizing the
  risk of conflict when two people add a new declaration for two
  different pins

Change-Id: I1ca0cc4f48bcd8cfd35b331e9821935f5c855876
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>