xtensa: tell git to ignore generated .dtb files

Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>
Signed-off-by: Chris Zankel <chris@zankel.net>

arch/xtensa/boot/.gitignore

@@ -1,2 +1,3 @@
 uImage
 zImage.redboot
+*.dtb