LPUART1 needs BRR to be 256 times the normal value.
Doing this with 64 bit math would work, but that pulls in quite a lot of
code for every platform, even if they don't have LPUART.
A little bit of maths, thanks to zyp on irc, lets us keep it in 32bit,
provided people don't try and use > 16MBaud on LPUARTs
Originally tracked via https://github.com/libopencm3/libopencm3/pull/888