[POWERPC] Add treeImage to .gitignore

Tell git to ignore the generated treeImage.* files in arch/powerpc/boot
Signed-off-by: NJosh Boyer <jwboyer@linux.vnet.ibm.com>

arch/powerpc/boot/.gitignore

@@ -19,6 +19,7 @@ kernel-vmlinux.strip.gz
 mktree
 uImage
 cuImage.*
+treeImage.*
 zImage
 zImage.bin.*
 zImage.chrp