[POWERPC] pasemi: Kconfig for gpio_mdio

arch/powerpc/platforms/pasemi/gpio_mdio.c really depends on CONFIG_PHYLIB.
Add a config option for it, allow for it to be disabled if needed and fix
the dependency.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>