serial: unify serial bindings into a single dir

Move all bindings in bindings/tty/serial into bindings/serial so we only
have one place dir with serial/uart related bindings in it.

Signed-off-by: Kumar Gala <galak@codeaurora.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>