ARM: tegra: support large RAM sizes

Some systems have so much RAM that the end of RAM is beyond 4GB. An
example would be a Tegra124 system (where RAM starts at 2GB physical)
that has more than 2GB of RAM.

In this case, we want gd->ram_size to represent the actual RAM size, so
that the actual RAM size is passed to the OS. This is useful if the OS
implements LPAE, and can actually use the "extra" RAM.

However, we can't use get_ram_size() to verify the actual amount of RAM
present on such systems, since some of the RAM can't be accesses, which
confuses that function. Avoid calling get_ram_size() when the RAM size
is too large for it to work correctly. It's never actually needed anyway,
since there's no reason for the BCT to report the wrong RAM size.

In systems with >=4GB RAM, we still need to clip the reported RAM size
since U-Boot uses a 32-bit variable to represent the RAM size in bytes.

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