dt-bindings: Fix multi pattern support in DT_SCHEMA_FILES

dt-bindings: Fix multi pattern support in DT_SCHEMA_FILES

DT_SCHEMA_FILES used to allow specifying a space separated list of file
paths, but the introduction of partial matches support broke this
feature:

$ make dtbs_check DT_SCHEMA_FILES="path/to/schema1.yaml path/to/schema2.yaml"
[...]
  LINT    Documentation/devicetree/bindings
usage: yamllint [-h] [-] [-c CONFIG_FILE | -d CONFIG_DATA] [--list-files] [...]
                [-v]
                [FILE_OR_DIR ...]
yamllint: error: one of the arguments FILE_OR_DIR - is required
[...]

Restore the lost functionality by preparing a grep filter that is able
to handle multiple search patterns.

Additionally, as suggested by Rob, use ':' instead of ' ' as the
patterns separator char. Hence, the command above becomes:

$ make dtbs_check DT_SCHEMA_FILES="path/to/schema1.yaml:path/to/schema2.yaml"

Fixes: 309d955985ee ("dt-bindings: kbuild: Support partial matches with DT_SCHEMA_FILES")
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20230209193735.795288-1-cristian.ciocaltea@collabora.com
Signed-off-by: Rob Herring <robh@kernel.org>