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