xtensa: tell git to ignore generated .dtb files

Signed-off-by: NBaruch Siach <baruch@tkos.co.il>
Signed-off-by: NMax Filippov <jcmvbkbc@gmail.com>
Signed-off-by: NChris Zankel <chris@zankel.net>

arch/xtensa/boot/.gitignore

 uImage
 zImage.redboot
+*.dtb