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: 90eff9096c01 ("net: phy: Allow splitting MDIO bus/device support from PHYs")
Fixes: 648ea0134069 ("net: phy: Allow pre-declaration of MDIO devices")
Signed-off-by: Arnd Bergmann <[e-mail]>
Signed-off-by: Gregory CLEMENT <[e-mail]>

arch/arm/mach-orion5x/Kconfig

···
 	select GPIOLIB
 	select MVEBU_MBUS
 	select PCI
+	select PHYLIB if NETDEVICES
 	select PLAT_ORION_LEGACY
 	help
 	  Support for the following Marvell Orion 5x series SoCs:

arch/arm/plat-orion/common.c

···
 		    eth_data, &orion_ge11);
 }
 
+#ifdef CONFIG_ARCH_ORION5X
 /*****************************************************************************
  * Ethernet switch
  ****************************************************************************/
···
 {
 	struct mdio_board_info *bd;
 	unsigned int i;
+
+	if (!IS_BUILTIN(CONFIG_PHYLIB))
+		return;
 
 	for (i = 0; i < ARRAY_SIZE(d->port_names); i++)
 		if (!strcmp(d->port_names[i], "cpu"))
···
 
 	mdiobus_register_board_info(&orion_ge00_switch_board_info, 1);
 }
+#endif
 
 /*****************************************************************************
  * I2C