riscv: dts: microchip: add missing CAN bus clocks

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 to the DT.

Fixes: 38a71fc0 ("riscv: dts: microchip: add mpfs's CAN controllers")
Signed-off-by: Conor Dooley <conor.dooley@microchip.com>