dt-bindings: can: mpfs: add missing required clock

The CAN controller on PolarFire SoC has an AHB peripheral clock _and_ a
CAN bus clock. The bus clock was omitted when the binding was written,
but is required for operation. Make up for lost time and add it.

Cautionary tale in adding bindings without having implemented a real
user for them perhaps.

Fixes: c878d518d7b6 ("dt-bindings: can: mpfs: document the mpfs CAN controller")
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>

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>