.gitignore: Ignore regenerated *.dtbo files

*.dtbo are dt overlays files which should be also ignored as *.dtb.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>
diff --git a/.gitignore b/.gitignore
index 8d18d6f..3df3139 100644
--- a/.gitignore
+++ b/.gitignore
@@ -10,6 +10,7 @@
 *.bin
 *.cfgout
 *.dtb
+*.dtbo
 *.dtb.S
 *.elf
 *.exe