kbuild: Cleanup DT Overlay intermediate files as appropriate

%.dtbo.o and %.dtbo.S files are used to build-in DT Overlay. They should
should not be removed by Make or the kernel will be needlessly rebuilt.

These should be removed by "clean" and ignored by git like other
intermediate files.

Reported-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Andrew Davis <afd@ti.com>
Fixes: 941214a512d8 ("kbuild: Allow DTB overlays to built into .dtbo.S files")
Tested-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20221114205939.27994-1-afd@ti.com
Signed-off-by: Rob Herring <robh@kernel.org>
diff --git a/.gitignore b/.gitignore
index 5da0048..3ec73ea 100644
--- a/.gitignore
+++ b/.gitignore
@@ -20,6 +20,7 @@
 *.dtb
 *.dtbo
 *.dtb.S
+*.dtbo.S
 *.dwo
 *.elf
 *.gcno