dt-bindings: net: can: add "st,can-secondary" property

On the stm32f7 Socs the can peripheral can be in single or dual
configuration. In the dual configuration, in turn, it can be in primary
or secondary mode. The addition of the 'st,can-secondary' property allows
you to specify this mode in the dual configuration.

CAN peripheral nodes in single configuration contain neither
"st,can-primary" nor "st,can-secondary".

Signed-off-by: Dario Binacchi <dario.binacchi@amarulasolutions.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/all/20230427204540.3126234-2-dario.binacchi@amarulasolutions.com
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>