ARM: tegra: p2771-000: increase max DRAM bank count

On this platform, there may be up to 1024 unusable chunks of memory.
Increase CONFIG_NR_DRAM_BANKS so that U-Boot can remember all the banks
required to represent such fragmented memory.

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