kbuild: fix build rule of u-boot-spl.dtb

kbuild: fix build rule of u-boot-spl.dtb

The build command of u-boot-spl.dtb is not constant, but dependent
on CONFIG_OF_SPL_REMOVE_PROPS.  Use $(call if_changed,...) so that
the change of CONFIG_OF_SPL_REMOVE_PROPS is detected.

Also, add tools/fdtgrep to the dependency to make sure u-boot-spl.dtb
is generated by the up-to-date fdtgrep in case the tool is modified.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Simon Glass <sjg@chromium.org>