can: rcar_can: support all input clocks

When writing the driver, I didn't give enough attention to the possible sources
of the CAN clock: although the value of the CLKR register was specified by the
platform data, the driver only handled one case, that is CAN clock being
sourced from the clkp1 clock, the same that clocks the whole CAN module. In
order to fix that overlook, we'll have to handle the CAN clock separately from
the peripheral clock (however, clkp1 will be specified for a CAN device only
once)...

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>