test: regmap: test Linux-compatible syscon_node_to_regmap()

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