Implement POWER7's CFAR in TCG

This patch implements support for the CFAR SPR on POWER7 (Come From
Address Register), which snapshots the PC value at the time of a branch or
an rfid.  The latest powerpc-next kernel also catches it and can show it in
xmon or in the signal frames.

This works well enough to let recent kernels boot (which otherwise oops
on the CFAR access).  It hasn't been tested enough to be confident that the
CFAR values are actually accurate, but one thing at a time.

Signed-off-by: Ben Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Alexander Graf <agraf@suse.de>
