powerpc: Update boot wrapper script with the new location of dtc

dtc was moved in 9fffb55f

 from
arch/powerpc/boot/ to scripts/dtc/

This patch updates the wrapper script to point to the new location of dtc.

Signed-off-by: Lucian Adrian Grijincu <lgrijincu@ixiacom.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>