docs: Makefile: Don't look for Makefiles in bindings

The device-tree bindings directory no longer contains the
Makefile that the Documentation/Makefile is looking for.
Hence, to avoid compilation issues, ignore the part where
it looks for the bindings' Makefile.

Change-Id: I6477f1b75697e0b9b9edf537b585a58d89dedfbf
Signed-off-by: Raghavendra Rao Ananta <rananta@codeaurora.org>