powerpc/configs: Update for CONFIG_FIXED_PHY being selected by CONFIG_OF_MDIO

In commit a5e4bd99 ("of_mdio: select fixed phy support
unconditionally"), CONFIG_OF_MDIO began selecting CONFIG_FIXED_PHY.

That means we no longer need to set it some of our defconfigs.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>