uboot-phytec

doc: move documentation for /config node into a separate file

This moves the description of the /config node from README.fdt-control
into a separate file doc/device-tree-bindings/config.txt.

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Simon Glass <sjg@chromium.org>