hardware_gen: add elfloader CPUs output

outputs all CPUs described in the DT to the elfloader
header and also includes any devices in the seL4,elfoader-devices
property.

tools/hardware.yml

@@ -238,3 +238,12 @@ devices:
     regions:
       - index: 0
         kernel: PLIC_PPTR
+
+  # elfloader rules
+  - compatible:
+      - arm,psci-0.2
+      - arm,psci-1.0
+  - compatible:
+      - fsl,imx6q-src
+  - compatible:
+      - xlnx,zynq-reset