pinctrl: move dm_scan_fdt_node() out of pinctrl uclass

Commit c5acf4a2 ("pinctrl: Add the concept of peripheral IDs")
added some additional change that was not mentioned in the git-log.

That commit added dm_scan_fdt_node() in the pinctrl uclass binding.
It should be handled by the simple-bus driver or the low-level
driver, not by the pinctrl framework.

I guess Simon's motivation was to bind GPIO banks located under the
Rockchip pinctrl device.  It is true some chips have sub-devices
under their pinctrl devices, but it is basically SoC-specific matter.

This commit partly reverts commit c5acf4a2 to keep the only
pinctrl-generic features in the uclass.  The dm_scan_fdt_node()
should be called from the rk3288_pinctrl driver.

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