fdtdec: Add weak function to patch U-Boot DT right after fdtdec_setup()

Add weak function which is called right after fdtdec_setup() configured
the U-Boot DT. This permits board-specific adjustments to the U-Boot DT
before U-Boot starts parsing the DT. This could be used e.g. to patch in
various custom nodes or merge in DT fragments from prior-stage firmware.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Cc: Simon Glass <sjg@chromium.org>
Cc: Tom Rini <trini@konsulko.com>