Separate DT code in preparation for follow-on ACPI integration.

Based on code from: Tomasz Nowicki <tomasz.nowicki@linaro.org>
Signed-off-by: NRobert Richter <rrichter@cavium.com>
Signed-off-by: NDavid Daney <david.daney@cavium.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>