.gitignore: move *.dtb and *.dtb.S patterns to the top-level .gitignore

Follow Linux commit 10b62a2f785a (".gitignore: move *.dtb and *.dtb.S
patterns to the top-level .gitignore").

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
diff --git a/.gitignore b/.gitignore
index 4d4c486..6bb0ade 100644
--- a/.gitignore
+++ b/.gitignore
@@ -9,6 +9,8 @@
 *.a
 *.bin
 *.cfgtmp
+*.dtb
+*.dtb.S
 *.elf
 *.exe
 *.gcda