Makefile: Use the fdtgrep -u flag

Use this flag so that the bootph binding is obeyed correctly.

Add a comment about what is going on.

Signed-off-by: Simon Glass <sjg@chromium.org>
Fixes: https://source.denx.de/u-boot/custodians/u-boot-dm/-/issues/12


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