net: mdio-octeon: Drop obsolete Kconfig advice

"Y" was the right answer for MDIO_OCTEON when this option was only
available on CAVIUM_OCTEON_SOC. But now that the option is visible on
all (64-bit) systems, this piece of advice no longer makes sense. This
helper module is selected automatically by drivers which need it
anyway.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Fixes: a6d67864 ("net: mdio-octeon: Modify driver to work on both ThunderX and Octeon")
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: Sunil Goutham <sgoutham@cavium.com>
Cc: Radha Mohan Chintakuntla <rchintakuntla@cavium.com>
Cc: David Daney <david.daney@cavium.com>
Cc: David S. Miller <davem@davemloft.net>
Acked-by: David Daney <david.daney@cavium.com>
Signed-off-by: David S. Miller <davem@davemloft.net>