can: mcp251xfd: ram: add helper function for runtime ring size calculation

This patch adds a helper function to calculate the ring configuration
of the controller based on various constraints, like available RAM,
size of RX and TX objects, CAN-mode, number of FIFOs and FIFO depth.

Link: https://lore.kernel.org/20220313083640.501791-3-mkl@pengutronix.de


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