scripts/Makefile.lib: Add dtsi include files as deps for building DTB

At the time of building the DTB, some dtsi files can be selected for
inclusion. Have these dtsi files as dependencies for the DTB
target. This also ensures generation or updating the dtsi files if
need be.

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Acked-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Reviewed-by: Tom Rini <trini@konsulko.com>