dt-bindings: ignore paths outside kernel for DT_SCHEMA_FILES

If the location of the kernel sources contains the string that we're
filtering for using DT_SCHEMA_FILES, then all schemas will currently be
matched, returned and checked, not just the ones we actually expected.
As an example, if the kernel sources happen to be below a directory
'google', and DT_SCHEMA_FILES=google, everything is checked. More
common examples might be having the sources below people's home
directories that contain the string st or arm and then searching for
those. The list is endless.

Fix this by only matching for schemas below the kernel source's
bindings directory.

Note that I opted for the implementation here so as to not having to
deal with escaping DT_SCHEMA_FILES, which would have been the
alternative if the grep match itself had been updated.

Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Link: https://lore.kernel.org/r/20231220145537.2163811-1-andre.draszik@linaro.org
Signed-off-by: Rob Herring <robh@kernel.org>