ARM: dts: keystone: Add minimal Keystone SOC device tree data

Add minimal device tree data for Keystone2 based SOCs. Patch
contains mainly ARM related SOC data and nothing about EVM specific
yet.

Cc: Grant Likely <grant.likely@linaro.org>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: arm@kernel.org

Acked-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Santosh Shilimkar <santosh.shilimkar@ti.com>