Add hwmod support to enable access to UART4 of the AM35xx series of
chips.  The UART4 device referenced from the TRM will show up as ttyO3.

This was tested on an AM3505.
Signed-off-by: NKyle Manna <kyle.manna@fuel7.com>
Signed-off-by: NPaul Walmsley <paul@pwsan.com>