dt-bindings: arm: hpe: add GXP Support

Add support for HPE GXP. The GXP is based on the cortex a9 processor and
supports arm7.

Signed-off-by: Nick Hawkins <nick.hawkins@hpe.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>