fdt_region: Ensure that depth never goes below -1

A specially crafted FIT image makes it possible to overflow the stack
with controlled values when using the verified boot feature. Depending
on the memory layout, this could be used to overwrite configuration
variables on the heap and setting them to 0, e.g. disable signature
verification, thus bypassing it.

This change fixes a bug in fdt_find_regions where the fdt structure is
parsed. A lower value than -1 of depth can lead to a buffer underflow
write on the stack.

Signed-off-by: Konrad Beckmann <konrad.beckmann@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>