can: mcp251xfd: mcp251xfd_chip_timestamp_init(): factor out into separate function

This patch factors out the timestamp initialization from the clock
initialization.

This is a preparation patch for the PLL support, where clock and
timestamp init must be done separately.

Link: https://lore.kernel.org/all/20220207131047.282110-11-mkl@pengutronix.de


Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>