eeprom-93cx6: Add (read-only) support for 8-bit mode

Add read-only support for EEPROMs configured in 8-bit mode (ORG pin connected
to GND).
This will be used by wd719x driver.

Signed-off-by: Ondrej Zary <linux@rainbow-software.org>
Reviewed-by: Hannes Reinecke <hare@suse.de>
Signed-off-by: Christoph Hellwig <hch@lst.de>