ARC: [plat-axs10x]: use pgu pll instead of fixed clock

Use PLL driver instead of fixed-clock for PGU pixel clock.
That allows us to support wider range of graphic modes.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>