ARM: shmobile: lager: phy fixup needs CONFIG_PHYLIB

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 48c8b96f21817aad
("ARM: shmobile: Lager: add Micrel KSZ8041 PHY fixup")

Cc: Sergei Shtylyov <[e-mail]>
Signed-off-by: Simon Horman <[e-mail]>

arch/arm/mach-shmobile/board-lager.c

···
 {
 	lager_add_standard_devices();
 
-	phy_register_fixup_for_id("r8a7790-ether-ff:01", lager_ksz8041_fixup);
+	if (IS_ENABLED(CONFIG_PHYLIB))
+		phy_register_fixup_for_id("r8a7790-ether-ff:01",
+					  lager_ksz8041_fixup);
 }
 
 static const char * const lager_boards_compat_dt[] __initconst = {