can: mcp25xxfd: mcp25xxfd_probe(): add SPI clk limit related errata information

This patch adds a reference to the recent released MCP2517FD and MCP2518FD
errata sheets and paste the explanation.

The driver already implements the proposed fix.

Signed-off-by: Thomas Kopp <thomas.kopp@microchip.com>
Link: https://lore.kernel.org/r/20200925065606.358-1-thomas.kopp@microchip.com


[mkl: split into two patches, adjust subject and commit message]
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>