soc: microchip: mpfs: add auto-update subdev to system controller

The PolarFire SoC's system controller offers the ability to re-program
the FPGA from a user application via two, related, mechanisms.
In-Application Programming (IAP) is not ideal for use in Linux, as it
will immediately take down the system when requested. Auto Update is
preferred, as it will only take affect at device power up*, allowing the
OS (and potential applications in AMP) to be shut down gracefully.

* Auto Update occurs at device initialisation, which can also be
  triggered by device reset - possible with the v2023.02 version of the
  Hart Software Services (HSS) and reference design.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>