dt-bindings: kbuild: Simplify examples target patsubst

Instead of stripping off the $(srctree) multiple times do it once up
front, but keep the src/obj path as it is going to be needed in
subsequent commit.

Rename the variable to CHK_DT_EXAMPLES to better reflect what it
contains.

Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>