dts: arm: Add base DTS and YAML definitions

This patch adds the base DTS and YAML files that may be referenced by
SoCs.  The device tree files provide base definitions for the more well
known ARMV7M device nodes.  These files are meant to be included in
SoC vendor specific device tree files.

The YAML files provide definitions for the contents of device tree node
that contain information that will be parsed and used for configuration
in the system.

DTS files define hardware and software configuration and YAML files
provide the markers for knowing which pieces need to be extracted.

Change-Id: I7e90fe19f09afb269b7b2988c2c19c0f26d7ee7c
Signed-off-by: Andy Gross <andy.gross@linaro.org>