drivers: can: sam: use UPLL clock instead of PLLA clock

Use the UPLLCK clock for the CAN controller as recommended by the Atmel SAM
E70 data sheet.

Move the configuration of the clock prescaler from Kconfig to devicetree
and limit it to the values recommended by the SAM E70 datasheet.

Fixes: #45012

Signed-off-by: Henrik Brix Andersen <henrik@brixandersen.dk>