powerpc/powernv/pci: Rework accessing the TCE invalidate register

It's architected, always in a known place, so there is no need
to keep a separate pointer to it, we use the existing "regs",
and we complement it with a real mode variant.

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

# Conflicts:
#	arch/powerpc/platforms/powernv/pci-ioda.c
#	arch/powerpc/platforms/powernv/pci.h
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>