cpu/stm32/periph/usbdev_fs: implement PMA access schemes
cpu/stm32/periph/usbdev_fs: implement PMA access schemes

There are two schemes for accessing the packet buffer area (PMA) from the CPU:
 - 2 x 16 bit/word access scheme where two 16-bit half-words per word can be accessed. With this scheme the access can be half-word aligned and the  PMA address offset corresponds therefore to the local USB IP address.  The size of the PMA SRAM is usually 1024 byte.
- 1 x 16 bit/word access scheme where one 16-bit half word per word can be  accessed. With this scheme the access can only be word-aligned and the  PMA address offset to a half-word is therefore twice the local USB IP  address. The size of the PMA SRAM is usually 512 byte.
Which access scheme is used depends on the STM32 model.
jasonatran/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,jasonatran/RIOT,jasonatran/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,kaspar030/RIOT,kaspar030/RIOT,RIOT-OS/RIOT,jasonatran/RIOT,ant9000/RIOT,ant9000/RIOT,RIOT-OS/RIOT,ant9000/RIOT,ant9000/RIOT,kaspar030/RIOT,jasonatran/RIOT,RIOT-OS/RIOT,ant9000/RIOT