MIPS: OCTEON: delete SMI/MDIO enable

SMI/MDIO enable is handled by the OCTEON MDIO driver, so we can delete
the duplicated functionality from the platform code.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: linux-mips@vger.kernel.org