soc: microchip: mpfs: enable access to the system controller's flash

The system controller has a flash that contains images used to reprogram
the FPGA using IAP (In-Application Programming).
Introduce a function that allows a driver with a reference to the system
controller to get one to a flash device attached to it.

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