Add fdt_find_and_setprop() to fdt_support.h

fdt_find_and_setprop() is used by several 4xx boards and it's
missing in the appropriate header. This patch eliminates a
warning when building U-Boot for such boards.

Signed-off-by: Matthias Fuchs <matthias.fuchs@esd-electronics.com>
Acked-by: Stefan Roese <sr@denx.de>