powerpc: Fix build dependencies for c files requiring libfdt.h

Several files in obj-plat depend on libfdt header file. Sometimes
when building one can see the following issue. This patch adds
libfdt as dependency to those object files

| In file included from arch/powerpc/boot/treeboot-iss4xx.c:33:0:
| arch/powerpc/boot/libfdt.h:854:1: error: unterminated comment
| In file included from arch/powerpc/boot/treeboot-iss4xx.c:33:0:
| arch/powerpc/boot/libfdt.h:1:0: error: unterminated #ifndef
|   BOOTCC  arch/powerpc/boot/inffast.o
| make[1]: *** [arch/powerpc/boot/treeboot-iss4xx.o] Error 1
| make[1]: *** Waiting for unfinished jobs....
|   BOOTCC  arch/powerpc/boot/inflate.o
| make: *** [uImage] Error 2
| ERROR: oe_runmake failed
| ERROR: Function failed: do_compile (see /srv/home/pokybuild/yocto-autobuilder/yocto-slave/p1022ds/build/build/tmp/work/p1022ds-poky-linux-gnuspe/linux-qoriq-sdk-3.0.34-r5/temp/log.do_compile.2167 for further information)
NOTE: recipe linux-qoriq-sdk-3.0.34-r5: task do_compile: Failed

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