ARM: dts: Add device tree files for hpe gxp soc

The HPE SoC is new to linux. A basic device tree layout with minimum
required for linux to boot including a timer and watchdog support has
been created.

The dts file is empty at this point but will be updated in subsequent
updates as board specific features are enabled.

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>