This patch adds RX IRQ coalescing support to the driver.

The mcp251xfd chip doesn't support proper hardware based coalescing,
so this patch tries to implemented it in software. The RX-FIFO offers
a "FIFO not empty" interrupt, which is used if no coalescing is
active.

With activated RX IRQ coalescing the "FIFO not empty" interrupt is
disabled in the RX IRQ handler and the "FIFO half full" or "FIFO full
interrupt" (depending on RX max coalesced frames IRQ) is used instead.
To avoid RX CAN frame starvation a hrtimer is setup with RX coalesce
usecs IRQ,on timer expiration the "FIFO not empty" is enabled again.

Support for ethtool configuration is added in the next patch.

Link: https://lore.kernel.org/20220313083640.501791-9-mkl@pengutronix.deSigned-off-by: Marc Kleine-Budde <mkl@pengutronix.de>