arm: dts: nrf: Add Device Tree Support for nRF51822 SoC & boards

Add device tree support for nRF51822 SoCs and Arduino 101-BLE,
Curie-BLE, BLE Nano, PCA10028-DK, and Quark-SE BLE boards. This
is minimal support for memory, flash, and UART.

Change-Id: I7e572bea537e384b6d66e520462f023ace0c9b35
Signed-off-by: Kumar Gala <kumar.gala@linaro.org>