include/ns16550.h: Unify structure declaration for registers

include/ns16550.h: Unify structure declaration for registers

Instead of special casing the different access patterns, use common
code with light macros sprinkled in to accomodate for the different
layouts of the register structure.

Note that this also changes the types of the registers for the
"positively packed (>1)" cases.  As the registers truly are unsigned
chars, this is surely the Right Thing, but it is a semantic change.
Note that for this case depending on the endianness on the bus, we may
see a change of behaviour.

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