pci: tegra: Fix port information parsing

commit a62e84d7 incorrectly changed the tegra pci code to the
new fdtdec pci helpers. To get the device index of the root port, the
"reg" property should be parsed from the dtb (as was previously the
case).

With this patch i can successfully network boot my jetson tk1

Signed-off-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Tested-by: Thierry Reding <treding@nvidia.com>
Acked-by: Thierry Reding <treding@nvidia.com>