dt-bindings: can: mpfs: document the mpfs CAN controller

Add a binding for the CAN controller on PolarFire SoC (MPFS).

A data sheet and a register map can be downloaded at:

| https://www.microsemi.com/document-portal/doc_download/1245725-polarfire-soc-fpga-mss-technical-reference-manual
| https://www.microsemi.com/document-portal/doc_download/1244581-polarfire-soc-register-map

An alternative location for the register map is:

| http://web.archive.org/web/20220403030214/https://www.microsemi.com/document-portal/doc_download/1244581-polarfire-soc-register-map

Link: https://lore.kernel.org/all/20220607065459.2035746-2-conor.dooley@microchip.com
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
[mkl: add link to data sheet and register map]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>