drm/fsl-dcu: add extra clock for pixel clock

The Vybrid DCU variant has two independent clock inputs, one
for the registers (IPG bus clock) and one for the pixel clock.
Support this distinction in the DCU DRM driver while staying
backward compatible for old device trees.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stefan Agner <stefan@agner.ch>