fdt: Add Kconfig options to control code size

For better or worse libfdt recent grew a lot of code that checks the
validity of the device tree in great detail. When using unsigned or
unverified data this makes things safer, but it does add to code size.

Add some controls to select the trade-off between safety and code size.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Tom Rini <trini@konsulko.com>