powerpc: Remove remaining iSeries chunks

In commit f5339277 "powerpc: Remove FW_FEATURE ISERIES from arch code", we
removed the bulk of the iSeries code, but missed a few bits.

Remove the mschunks bits, these were only ever used on iSeries as far as I
know, and are definitely not used anymore.

Make it even clearer that phys_to_abs() is a nop, by making it a macro. We
still have a few users of this, but should clean those up.

Signed-off-by: Michael Ellerman <michael@ellerman.id.au>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>