serial: stm32: Use macro to simplify registration

The registration of each serial port differs only by a few details.
These differences can be factorized in order to create a generic
registration macro.

This has several advantages:
- Less code
- Easier to add new ports
- Less work to add support for new STM32 families

Change-Id: I6e62a96ccbbf03c9d51bc2617db6a851ff0d83c7
Signed-off-by: Florian Vaussard <florian.vaussard@heig-vd.ch>