treewide: fdt: Move fdt_get_config_... to ofnode_conf_read...

The current API is outdated as it requires a devicetree pointer.

Move these functions to use the ofnode API and update this globally. Add
some tests while we are here.

Correct the call in exynos_dsim_config_parse_dt() which is obviously
wrong.

Signed-off-by: Simon Glass <sjg@chromium.org>