ARM: at91: Fix link breakage when !CONFIG_PHYLIB

Fixes:
arch/arm/mach-at91/built-in.o: In function `ksz9021rn_phy_fixup':
:(.text+0x1174): undefined reference to `mdiobus_write'
:(.text+0x1188): undefined reference to `mdiobus_write'
:(.text+0x119c): undefined reference to `mdiobus_write'
:(.text+0x11b0): undefined reference to `mdiobus_write'
arch/arm/mach-at91/built-in.o: In function `sama5_dt_device_init':
:(.init.text+0x1e34): undefined reference to `phy_register_fixup_for_uid'

when CONFIG_PHYLIB is not selected.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>