arm64: dts: ls1028a: Add properties node for Display output pixel clock

The LS1028A has a clock domain PXLCLK0 used for the Display output
interface in the display core, independent of the system bus frequency,
for flexible clock design. This display core has its own pixel clock.

This patch enable the pixel clock provider on the LS1028A.

Signed-off-by: Wen He <wen.he_1@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>