net: thunder: Factor out DT specific code in BGX

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

Based on code from: Tomasz Nowicki <tomasz.nowicki@linaro.org>

Signed-off-by: Robert Richter <rrichter@cavium.com>
Signed-off-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>