ARM: imx: mach-imx6ul: Fix allmodconfig build

We should call phy_register_fixup_for_uid() only when CONFIG_PHYLIB
is built-in, otherwise we get the following link error when building
allmodconfig:

arch/arm/mach-imx/built-in.o: In function `imx6ul_init_machine':
:(.init.text+0xa714): undefined reference to `phy_register_fixup_for_uid'

This is the same approach done in mach-imx6q.c and mach-imx6sx.c.

Reported-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawnguo@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>