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>