dt-bindings: soc/microchip: add info about services to mpfs sysctrl

The services actually provided by the system controller are not
documented so add some words about what the system controller can
actually do. Add a link to the oneline documentation with the specific
details of each individual service.
Also, drop the unneeded label from the example.
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>

Documentation/devicetree/bindings/soc/microchip/microchip,mpfs-sys-controller.yaml

@@ -10,9 +10,14 @@ maintainers:
   - Conor Dooley <conor.dooley@microchip.com>
 
 description: |
-  The PolarFire SoC system controller is communicated with via a mailbox.
-  This document describes the bindings for the client portion of that mailbox.
+  PolarFire SoC devices include a microcontroller acting as the system controller,
+  which provides "services" to the main processor and to the FPGA fabric. These
+  services include hardware rng, reprogramming of the FPGA and verfification of the
+  eNVM contents etc. More information on these services can be found online, at
+  https://onlinedocs.microchip.com/pr/GUID-1409CF11-8EF9-4C24-A94E-70979A688632-en-US-1/index.html
 
+  Communication with the system controller is done via a mailbox, of which the client
+  portion is documented here.
 
 properties:
   mboxes:
@@ -29,7 +34,7 @@ additionalProperties: false
 
 examples:
   - |
-    syscontroller: syscontroller {
+    syscontroller {
       compatible = "microchip,mpfs-sys-controller";
       mboxes = <&mbox 0>;
     };