ppc: Fix macio ESCC legacy mapping

ppc: Fix macio ESCC legacy mapping

The current mapping, while correct for the base ports (which is all the
driver uses these days), is wrong for the extended registers.

I suspect the bugs come from incorrect tables in the CHRP IO Ref document,
I have verified the new values here match Apple's MacTech.pdf.

Note: Nothing that I know of actually uses these registers so it's not a
huge deal, but this patch has the added advantage of adding comments to
document what the registers are.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>