arm: dts: nrf: Add Device Tree Support for nRF52832 SoC based boards

Add device tree support for Nitrogen 96board, BLE Nano 2, and
nRF52-PCA10040 DK boards.  This is minimal support for memory, flash,
and UART.

For the nRF52832 we select between "nordic,nrf-uarte", "nordic,nrf-uart"
support for each board.

Change-Id: Ia247b9b710a72416e9ab0de3ca1429bfab8917f8
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>