ARM: Tegra186: search for best RAM bank

In the future, the list of DRAM regions passed to U-Boot in the DTB may
be quite long and fragmented. Due to this, U-Boot must search through the
regions to find the best region to relocate into, rather than relying on
the current assumption that the top of bank 0 is a reasonable relocation
target. This change implements such searching.

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