of: add support for devicetree overlays ...

The devicetree overlay support is based on the Linux driver for device
tree overlays, but many features that are not required in Barebox are
left out.

Unlike Linux, which applies the overlay to the live devicetree, Barebox
registers a fixup for the overlay which is applied with other fixups to
whatever tree is fixed. This is necessary to apply the overlay to
devicetrees that are passed to Linux, which might differ from the
devicetree that is currently live in Barebox.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>