fix DaVinci NS16550_REG_SIZE regression

fix DaVinci NS16550_REG_SIZE regression

Update the DaVinci DM6446 boards to use the new convention
for CONFIG_SYS_NS16550_REG_SIZE ... the size hasn't changed
from the original 4 bytes, but these chips are little-endian.

(Resolves a regression added recently by the include/ns16550.h
patch to "Unify structure declaration for registers".  The code
previously worked just fine because the registers were accessed
as host-endian words, not as bytes.)

Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>