ARM: dts: imx7d: add node for PXP

The i.MX7d contains a Pixel Pipeline in version 3.0. Add the device tree
node to make it available.

Signed-off-by: Michael Tretter <m.tretter@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>