Rename common ns16550 constants with UART_ prefix to prevent conflicts

Fix problems introduced in commit
7b5611cd [inka4x0: Add hardware
diagnosis functions for inka4x0] which redefined MSR_RI which is
already used on PowerPC systems.

Also eliminate redundant definitions in ps2mult.h.  More cleanup will
be needed for other redundant occurrences though.

Signed-off-by: Detlev Zundel <dzu@denx.de>