arm: dts: Add DTS support for NRF52832 SoC

Add plumbing to build system and SoC level dtsi for the NRF52832 SoC.
We additionally add the necessary yaml files for the UART on the NRF52
SoCs.

Change-Id: I3b4a821b2993827e33d8e84bdbbc759d1521f8bd
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>