powerpc: dtc is required to build dtb files

Fixes this following:

$ make distclean; make corenet32_smp_defconfig; make p4080ds.dtb
  CLEAN   arch/powerpc/boot
  CLEAN   scripts/basic
  CLEAN   scripts/dtc
  CLEAN   scripts/genksyms
  CLEAN   scripts/kconfig
  CLEAN   scripts/mod
  CLEAN   scripts
  CLEAN   include/config include/generated
  CLEAN   .config
  HOSTCC  scripts/basic/fixdep
  HOSTCC  scripts/kconfig/conf.o
  SHIPPED scripts/kconfig/zconf.tab.c
  SHIPPED scripts/kconfig/zconf.lex.c
  SHIPPED scripts/kconfig/zconf.hash.c
  HOSTCC  scripts/kconfig/zconf.tab.o
  HOSTLD  scripts/kconfig/conf
scripts/kconfig/conf --silentoldconfig Kconfig
  DTC     arch/powerpc/boot/p4080ds.dtb
/bin/sh: /local/home/mattsm/git/linux/scripts/dtc/dtc: No such file or directory
make[1]: *** [arch/powerpc/boot/p4080ds.dtb] Error 1
make: *** [p4080ds.dtb] Error 2

Signed-off-by: Matthew McClintock <msm@freescale.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>