Add overlay for Seeed Studio CAN BUS FD HAT (#4034)

This patch adds the overlay for the Seeed Studio CAN BUS FD HAT
with two CAN FD Channels and an RTC.
https://www.seeedstudio.com/CAN-BUS-FD-HAT-for-Raspberry-Pi-p-4742.html



The overlay was generated by
ovmerge -c mcp251xfd-overlay.dts,spi0-0,interrupt=25 \
           mcp251xfd-overlay.dts,spi0-1,interrupt=24 \
           i2c-rtc-overlay.dts,pcf85063


Also, add description on how to generate overlays

Signed-off-by: Patrick Menschel <menschel.p@posteo.de>