libfdt: Add support for disabling internal checks

If libfdt returns -FDT_ERR_INTERNAL that generally indicates a bug in the
library. Add a new assumption for these cases since it should be save to
disable these checks regardless of the input.

Signed-off-by: Simon Glass <sjg@chromium.org>
Suggested-by: David Gibson <david@gibson.dropbear.id.au>
Message-Id: <20200302190255.51426-3-sjg@chromium.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>