drivers: can: add triple sampling mode flag

Add flag for enabling triple sampling of the CAN RX signal.

Normally just one sample of the RX signal is performed by the CAN
controller, but in triple sampling mode the RX signal is sampled three
times. These three samples are then used for determining the value of the
received bit, typically by majority vote.

Signed-off-by: Henrik Brix Andersen <hebad@vestas.com>