arm: dts: ti_lm3s6965: Add device tree support for Stellaris UART

Converted Stellaris UART driver over to utilize device tree generated
defines.  Added a yaml description for the uart, and converted over the
ti_lm3s6965 SoC & qemu_cortex_m3 board port over to utilize it.

Change-Id: Ie20844eb63d2c68eb59ad4160f7f5b5a35e2943b
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>