ARM: dts: imxrt1050-pinfunc: Add pinctrl binding header

Add binding header for i.MXRT1050 pinctrl device tree.

Signed-off-by: Giulio Benetti <giulio.benetti@benettiengineering.com>
Signed-off-by: Jesse Taube <Mr.Bossman075@gmail.com>
[Jesse: move pinfunc from dt-bindings to dts folder]
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>