* introduce a DebugUART baseclass,
* rework 8250 and PL011 implementations from kallisti5 to inherit DebutUART,
* each arch should override the IO methods to access registers.
* on ARM registers are 32bit-aligned.
* U-Boot still works for the verdex target.
* rPi still compiles, needs testing.
* Still some more consolidation needed to allow runtime choice of the UART type (as read from FDT blobs for ex.).
* serial.cpp should probably mostly be made generic as well.
* didn't touch x86 or ppc yet.
