test: dm: fdtdec: Validate FDT size in unit test

test: dm: fdtdec: Validate FDT size in unit test

The current FDT decoding tests calculate the memory required
for FDT manipulation by directly adding a fixed margin to
fdt_totalsize(gd->fdt_blob). The static analyzer flagged
"gd->fdt_blob->totalsize" as a tainted value being passed
to fdt_open_into().

Ensure the size is validated by checking that the total size
is within a reasonable maximum FDT limit for unit tests.

Signed-off-by: Adriana Nicolae <adriana@arista.com>