overlays: seeed-can-fd-hat: clarify how to identify HAT version

It turns out the used CAN SPI chip is not a good way to identify the version of
the CAN HAT.

There are two different board layouts of the Seeed Studio CAN BUS FD HAT. The
v1 board doesn't have a battery holder, while the v2 board has. Update the
overlay README accordinly.

Link: https://github.com/Seeed-Studio/seeed-linux-dtoverlays/issues/13


Cc: Patrick Menschel <menschel.p@posteo.de>
Signed-off-by: Marc Kleine-Budde <mkl@pengutronix.de>