MLK-16879-3 can: flexcan: add notes for avaliable fd mode rate combinations

MLK-16879-3 can: flexcan: add notes for avaliable fd mode rate combinations

CAN FD can only support some specific bitrate combinations in FD mode
due to HW limitations. See below info mentioned in RM:
"To minimize errors when processing FD frames, use the same value
for FPRESDIV and PRESDIV (in CAN_CBT or CAN_CTRL1)."

Add notes in driver to let user know easily:

CAN FD supported rates combinations
* Combination 1:
*  Bitrate: 225000 375000 400000 425000 500000 875000
*  Data rate: 1000000
*
* Combination 2:
*  Bitrate:   550000 600000 625000 650000 675000 750000 775000
*             800000 850000 925000 950000 975000 1000000
*  Data rate: 1500000 2000000 2500000 3000000 3500000 4000000
*             5000000

Acked-by: Jason Liu <jason.hui.liu@nxp.com>
Signed-off-by: Dong Aisheng <aisheng.dong@nxp.com>