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>

clk: microchip: mpfs: add reset controller

Add a reset controller to PolarFire SoC's clock driver. This reset
controller is registered as an aux device and read/write functions
exported to the drivers namespace so that the reset controller can
access the peripheral device reset register.

Reviewed-by: Daire McNamara <daire.mcnamara@microchip.com>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Link: https://lore.kernel.org/r/20220909123123.2699583-5-conor.dooley@microchip.com

soc: add microchip polarfire soc system controller

This driver provides an interface for other drivers to access the
functions of the system controller on the Microchip PolarFire SoC.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@microchip.com>
Link: https://lore.kernel.org/r/20220217101349.2374873-2-conor.dooley@microchip.com

mbox: add polarfire soc system controller mailbox

This driver adds support for the single mailbox channel of the MSS
system controller on the Microchip PolarFire SoC.

Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>