PPC: e500: require libfdt

PPC: e500: require libfdt

Now that we're moving all of the device tree generation from an external
pre-execution generated blob to runtime generation using libfdt, we absolutely
must have libfdt around.

This requirement was there before already, as the only way to not require libfdt
with e500 was to not use -kernel, which was the only way to boot the mpc8544ds
machine. This patch only manifests said requirement in the build system.

Signed-off-by: Alexander Graf <agraf@suse.de>