riscv: dts: microchip: add mpfs's CAN controllers

PolarFire SoC has a pair of CAN controllers, but as they were
undocumented there were omitted from the device tree. Add them.

Link: https://lore.kernel.org/all/20220607065459.2035746-3-conor.dooley@microchip.com
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>