powerpc: Have git ignore generated files from dtc compile

Have git ignore generated files from dtc compile

Signed-off-by: Jon Smirl <jonsmirl@gmail.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Sean MacLennan <smaclennan@pikatech.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>