Let's make the code more common for gpios.

The only difference between STM32 families is that BRR register may
not be available. In case BRR is not available, we use the 16 left bits
of BSRR instead. We could always use BSRR, but BRR saves one left-shift
operation, so let's use it when available.

By default we will consider using BRR, except for platforms that define
GPIO_IP_WITHOUT_BRR.