ARM: imx6q: register phy fixup only when CONFIG_PHYLIB is enabled

It fixes the following compile error with network disabled in
imx_v6_v7_defconfig.

arch/arm/mach-imx/built-in.o: In function `ksz9021rn_phy_fixup':
imx53-dt.c:(.text+0x5f70): undefined reference to `mdiobus_write'
imx53-dt.c:(.text+0x5f84): undefined reference to `mdiobus_write'
imx53-dt.c:(.text+0x5f98): undefined reference to `mdiobus_write'
imx53-dt.c:(.text+0x5fac): undefined reference to `mdiobus_write'
imx53-dt.c:(.text+0x5fc0): undefined reference to `mdiobus_write'
arch/arm/mach-imx/built-in.o: In function `imx6q_init_machine':
imx53-dt.c:(.init.text+0x387c): undefined reference to `phy_register_fixup_for_uid'
make: *** [.tmp_vmlinux1] Error 1

Reported-by: Artem Bityutskiy <dedekind1@gmail.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>