uboot-phytec

USBTTY make some function declarations easier to use.

Zoom2 needs to use these declarations and the include directory is a
better place from them than in the middle of the driver directory.
It did not make sense to create a new file for just a couple of
lines so they were appended to the serial.h

Signed-off-by: Tom Rix <Tom.Rix@windriver.com>