ignore dtbo files

Change-Id: I0b75e802b7f1e0d995b9f6cccb24c2351c0ce60b
Reviewed-on: https://tp-biosrd-v02/gerrit/80078
Reviewed-by: Jamess Huang(黃以民) <Jamess_Huang@asus.com>
Tested-by: Jamess Huang(黃以民) <Jamess_Huang@asus.com>

Author: jamess-huang

arch/arm/boot/.gitignore

@@ -4,4 +4,5 @@ xipImage
 bootpImage
 uImage
 *.dtb
-zImage-dtb
+*.dtbo
+zImage-dtb