dt-bindings: mfd: syscon: Document the non simple-mfd syscon on PolarFire SoC

The "mss_top_scb" register region on PolarFire SoC contains many
different functions, including controls for the AXI bus and other things
mainly of interest to the bootloader. The interrupt register for the
system controller's mailbox is also in here, which is needed by the
operating system.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Acked-by: Rob Herring (Arm) <robh@kernel.org>
Link: https://lore.kernel.org/r/20241002-clambake-raider-a8cbb3a021a8@spud


Signed-off-by: Lee Jones <lee@kernel.org>