powerpc/ep8248e: Unconditionally select PHYLIB for mdio_bitbang

Its possible to build the phylib as a module, however this breaks the
board code because alloc_mdio_bitbang and mdiobus_register are not
available if we build as a module.  These are needed by the board code
since it implements the low level mdio bitbang ops.

So we unconditionally select PHYLIB to ensure its built into the kernel
if we are building in EP8248E support.

Long term we should look at moving the mdio_ops into its own file so it
can be built as a module.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>