Do not build the phy fixup unless CONFIG_PHYLIB is enabled.

Other than not being useful it is also not possible to link
the code under this condition as phy_register_fixup_for_id(),
mdiobus_read() and mdiobus_write() are absent.

arch/arm/mach-shmobile/built-in.o: In function `lager_ksz8041_fixup':
board-lager.c:(.text+0xb8): undefined reference to `mdiobus_read'
board-lager.c:(.text+0xd4): undefined reference to `mdiobus_write'
arch/arm/mach-shmobile/built-in.o: In function `lager_init':
board-lager.c:(.init.text+0xafc): undefined reference to `phy_register_fixup_for_id'

This problem was introduced by 48c8b96f
("ARM: shmobile: Lager: add Micrel KSZ8041 PHY fixup")

Cc: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>