Allow com(4) to make use of the simplebus address translation.

Allow com(4) to make use of the simplebus address translation.
This typically works for every simplebus client, but com(4) did not
make use of the supplied bus tag.  Instead it references the global
a4x bus tag.  This is needed as the ARM com(4) controllers use 4-byte
spaced registers, while com(4) operates on 1-byte.  To be able to make
use of the address translation, copy the a4x bus tag and replace the
cookie and map function with the one supplied by simplebus.

ok kettenis@