ARC: [plat-axs10x]: PGU: remove unused encoder-slave property

ARC PGU is looking for encoder via endpoint mechanism and doesn't
use "encoder-slave" property for a long time. Let's drop unused
"encoder-slave" property from ARC PGU node in axs10x.

Acked-by: Alexey Brodkin <[e-mail]>
Signed-off-by: Eugeniy Paltsev <[e-mail]>
Signed-off-by: Vineet Gupta <[e-mail]>

arch/arc/boot/dts/axs10x_mb.dtsi

···
 		pgu@17000 {
 			compatible = "snps,arcpgu";
 			reg = <0x17000 0x400>;
-			encoder-slave = <&adv7511>;
 			clocks = <&pguclk>;
 			clock-names = "pxlclk";
 			memory-region = <&frame_buffer>;