powerpc: Better split CONFIG_PPC_INDIRECT_PIO and CONFIG_PPC_INDIRECT_MMIO

Remove the generic PPC_INDIRECT_IO and ensure we only add overhead
to the right accessors. IE. If only CONFIG_PPC_INDIRECT_PIO is set,
we don't add overhead to all MMIO accessors.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>