devicetree: fix name of pinctrl-bindings.txt

Rename:
	pinctrl-binding.txt -> pinctrl-bindings.txt

In order to match the current name of this file.

Signed-off-by: Mauro Carvalho Chehab <mchehab+samsung@kernel.org>
Acked-by: Jonathan Corbet <corbet@lwn.net>