ARM: orion5x: only call into phylib when available

Board code cannot call mdiobus_register_board_info() when phylib
or mdio_device is a loadable module:

arch/arm/plat-orion/common.o: In function `orion_ge00_switch_init':
:(.init.text+0x474): undefined reference to `mdiobus_register_board_info'

I had a number of ideas for how this could be solved, but after the MDIO
code got split out from PHYLIB it has gotten too hard, so I'm basically
giving up, and only call the mdiobus_register_board_info() function
if the MDIO layer is built-in to avoid the link error. This is similar
to how we handle PHY registration on other ARM platforms.

Fixes: 90eff909 ("net: phy: Allow splitting MDIO bus/device support from PHYs")
Fixes: 648ea013 ("net: phy: Allow pre-declaration of MDIO devices")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>