BeagleBone Blue: usarts: enable all the dedicated usart connectors.

DSM2/uart4: on the Blue, this is a recieve only interface.
Signed-off-by: NRobert Nelson <robertcnelson@gmail.com>
Cc: Jason Kridner <jkridner@beagleboard.org>
Cc: Drew Fustini <drew@beagleboard.org>
Signed-off-by: NTony Lindgren <tony@atomide.com>