bindings: arm: fsl: Add PHYTEC i.MX8MP devicetree bindings

Add devicetree bindings for i.MX8MP based phyCORE-i.MX8MP
and phyBOARD-Pollux RDK.

Signed-off-by: Teresa Remmet <t.remmet@phytec.de>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>