ARM: tegra: pick up actual memory size

On Tegra186, U-Boot is booted by the binary firmware as if it were a
Linux kernel. Consequently, a DTB is passed to U-Boot. Cache the address
of that DTB, and parse the /memory/reg property to determine the actual
RAM regions that U-Boot and subsequent EL2/EL1 SW may actually use.

Given the binary FW passes a DTB to U-Boot, I anticipate the suggestion
that U-Boot use that DTB as its control DTB. I don't believe that would
work well, so I do not plan to put any effort into this. By default the
FW-supplied DTB is the L4T kernel's DTB, which uses non-upstreamed DT
bindings. U-Boot aims to use only upstreamed DT bindings, or as close as
it can get. Replacing this DTB with a DTB using upstream bindings is
physically quite easy; simply replace the content of one of the GPT
partitions on the eMMC. However, the binary FW at least partially relies
on the existence/content of some nodes in the DTB, and that requires the
DTB to be written according to downstream bindings. Equally, if U-Boot
continues to use appended DTBs built from its own source tree, as it does
for all other Tegra platforms, development and deployment is much easier.

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

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