Like Linux, syscon_node_to_regmap() allows a node to work as a syscon
provider without binding it to a syscon driver.  Test this.
Requested-by: NSimon Glass <sjg@chromium.org>
Signed-off-by: NMasahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: NSimon Glass <sjg@chromium.org>