ARM: Tegra186: mem parsing fixes from downstream

Apply a few small fixes for the DTB /memory node parsing from NVIDIA's
downstream U-Boot:

- Allow arbitrary number of DRAM banks.
- Correctly calculate the number of DRAM banks.
- Clip PCIe memory in the same way as U-Boot CPU memory use.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Tom Warren <twarren@nvidia.com>