dt-binding: can: mcp25xxfd: document device tree bindings

This patch adds the device-tree binding documentation for the Microchip
MCP25xxFD SPI CAN controller family.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>
Link: https://lore.kernel.org/r/20200918172536.2074504-2-mkl@pengutronix.de