arm: dts: nrf: Add Device Tree Support for nRF52840 SoC & boards

Add device tree support for nRF52840 SoC and PCA10056-DK board.  This
is minimal support for memory, flash, and UART.

For the nRF52840 we select between "nordic,nrf-uarte", "nordic,nrf-uart"
support for each board.

Change-Id: I1c377e0cb97ff4716ea5489fffaa7c0e2b34d18a
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>