bcmstb: Make use of bootm_size rather than fdt_high

bcmstb: Make use of bootm_size rather than fdt_high

Remove setting of fdt_high to ~0, which disables device tree relocation,
from the default environment. Doing so prevents U-Boot from correcting
problems such as having an unaligned device tree and leads to various
failure modes in the OS. However, this platform also has a large comment
block that explains that given previous stage loaders and other parts of
the memory map (that may not be in the device tree we see?), adjust this
to use bootm_size to restrict relocation to be below the CMA area and
update the comment to match.

Signed-off-by: Tom Rini <trini@konsulko.com>