dt-bindings: fix error in 'make clean' after 'make dt_binding_check'

dt-bindings: fix error in 'make clean' after 'make dt_binding_check'

We are having more and more schema files.

Commit 8b6b80218b01 ("dt-bindings: Fix command line length limit
calling dt-mk-schema") fixed the 'Argument list too long' error of
the schema checks, but the same error happens while cleaning too.

'make clean' after 'make dt_binding_check' fails as follows:

  $ make dt_binding_check
    [ snip ]
  $ make clean
  make[2]: execvp: /bin/sh: Argument list too long
  make[2]: *** [scripts/Makefile.clean:52: __clean] Error 127
  make[1]: *** [scripts/Makefile.clean:66: Documentation/devicetree/bindings] Error 2
  make: *** [Makefile:1763: _clean_Documentation] Error 2

'make dt_binding_check' generates so many .example.dts, .dt.yaml files,
which are passed to the 'rm' command when you run 'make clean'.

I added a small hack to use the 'find' command to clean up most of the
build artifacts before they are processed by scripts/Makefile.clean

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Link: https://lore.kernel.org/r/20200625170434.635114-2-masahiroy@kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>