[platform][stm32f7xx][uart] add support for more than one active uart
[platform][stm32f7xx][uart] add support for more than one active uart

At the moment still only has hard coded bits for uart 1 and 3. Things
are declared in a wonky way to avoid allocating large structures unnecessarily.

hollanderic/lkstuff,travisg/lk,ErikCorryGoogle/lk,sndnvaps/lk,travisg/lk,minglun-tsai/lk,travisg/lk,ErikCorryGoogle/lk,littlekernel/lk,ErikCorryGoogle/lk,hollanderic/lkstuff,hollanderic/lkstuff,littlekernel/lk,minglun-tsai/lk,sndnvaps/lk,travisg/lk,sndnvaps/lk,littlekernel/lk,sndnvaps/lk,littlekernel/lk,sndnvaps/lk,minglun-tsai/lk,ErikCorryGoogle/lk,minglun-tsai/lk,hollanderic/lkstuff