TI816X: Add low level debug support

This patch adds support for low level debugging on TI816X boards. Currently the
support for UART3 console on TI816X EVM is added.

Signed-off-by: Hemant Pedanekar <hemantp@ti.com>
Reviewed-by: Kevin Hilman <khilman@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>