dt-bindings: don't anchor DT_SCHEMA_FILES to bindings directory

Commit 5e3ef454

 ("dt-bindings: ignore paths outside kernel for
DT_SCHEMA_FILES") anchored all searches to the bindings directory
(since bindings only exist below that), but it turns out this is not
always desired.

Just anchor to the base kernel source directory and while at it, break
the overly long line for legibility.

Reported-by: Michal Simek <michal.simek@amd.com>
Fixes: 5e3ef454

 ("dt-bindings: ignore paths outside kernel for DT_SCHEMA_FILES")
Closes: https://lore.kernel.org/all/827695c3-bb33-4a86-8586-2c7323530398@amd.com/
Cc: Masahiro Yamada <masahiroy@kernel.org>
Signed-off-by: André Draszik <andre.draszik@linaro.org>
Tested-by: Michal Simek <michal.simek@amd.com>
Link: https://lore.kernel.org/r/20240116062731.2810067-1-git@andred.net
Signed-off-by: Rob Herring <robh@kernel.org>