gpu: ipu-v3: add DT binding for the Prefetch Resolve Gasket

This adds the the devicetree binding for the Prefetch Resolve Gasket,
as found on i.MX6 QuadPlus.
The PRG is fairly simple in that it only has a configuration register
range and two clocks, one for the AHB slave port and one for the AXI
ports and the functional units.

The PRE connections need to be described in the DT, as the PRE<->PRG
assignment is a mix between fixed and muxable connections.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>