Setting DT_SCHEMA_FILES allows the user to restrict the
"dt_binding_check" make target to a specified set of DT binding files.
However, yamllint is still run on all available files, which not only
takes time, but also outputs warnings for other binding files the
developer is not interested in.

Fix this by renaming "find_cmd" to "find_all_cmd", introducing a new
"find_cmd" to only return the files specified by DT_SCHEMA_FILES (if
present), and using the latter for yamllint and dt-doc-validate.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/174ab1d791b7bc65f3b0f11b72be13af1748c731.1634551582.git.geert+renesas@glider.be
[robh: Also use only DT_SCHEMA_FILES for dt-doc-validate]
Signed-off-by: Rob Herring <robh@kernel.org>