ARM: tegra: trimslice: enable PCIe when booting from device tree

There currently aren't bindings for the Tegra PCIe controller. Work on
this is in progress, but not yet complete. Manually initialize PCIe when
booting from device tree, in order to bring DT support to the same
feature level as board files, which will in turn allow board files to be
deprecated.

PCIe hosts the wired Ethernet controller on TrimSlice.

To support this, add infra-structure to board-dt-tegra20.c for board-
specific initialization code. Once device tree support for the relevant
features is in place, this code will be removed.

Signed-off-by: Stephen Warren <swarren@nvidia.com>