dm: use list_count_nodes() for counting list nodes

The linux kernel has the list_count_nodes() API functions which is
used for counting nodes of a list. This has now been imported in
U-Boot as part of an earlier commit. Use this function and drop the
list_count_items().

Signed-off-by: Sughosh Ganu <sughosh.ganu@linaro.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>