ARM64: zynqmp: Support for multiple PM IDs assigned to a PM domain

Previously, it was assumed that there is a 1:1 mapping between
PM ID defined in the platform firmware and a PM domain. However, there
can be a situation where multiple PM IDs belong to a single PM domain
(e.g. PM IDs for GPU and two pixel processors correspond to a single
PM domain).

This patch adds support for assigning more than one PM ID to
a single PM domain.

Updated documentation accordingly.

Assigned pixel processors PM IDs to GPU PM domain.

Signed-off-by: Filip Drazic <filip.drazic@aggios.com>
Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/dts/zynqmp.dtsi

@@ -197,7 +197,7 @@
 
 		pd_gpu: pd-gpu {
 			#power-domain-cells = <0x0>;
-			pd-id = <0x3a>;
+			pd-id = <0x3a 0x14 0x15>;
 		};
 	};