Merge the changes to retire the legacy WR sbc8548 and sbc8641 platforms
from Paul. These were sent as a pull request, but I rebased them onto
rc2 so as not to pull too many unrelated changes in to my next.

Description from Paul's pull request follows:

In v2.6.27 (2008, 917f0af9) the sbc8260 support was implicitly
retired by not being carried forward through the ppc --> powerpc
device tree transition.

Then, in v3.6 (2012, b048b4e1) we retired the support for the
sbc8560 boards.

Next, in v4.18 (2017, 3bc6cf5a) we retired the support for the
2006 vintage sbc834x boards.

The sbc8548 and sbc8641d boards were maybe 1-2 years newer than the
sbc834x boards, but it is also 3+ years later, so it makes sense to
now retire them as well - which is what is done here.

These two remaining WR boards were based on the Freescale MPC8548-CDS
and the MPC8641D-HPCN reference board implementations.  Having had the
chance to use these and many other Fsl ref boards, I know this:  The
Freescale reference boards were typically produced in limited quantity
and primarily available to BSP developers and hardware designers, and
not likely to have found a 2nd life with hobbyists and/or collectors.

It was good to have that BSP code subjected to mainline review and
hence also widely available back in the day. But given the above, we
should probably also be giving serious consideration to retiring
additional similar age/type reference board platforms as well.

I've always felt it is important for us to be proactive in retiring
old code, since it has a genuine non-zero carrying cost, as described
in the 930d52c0 merge log.  But for the here and now, we just
clean up the remaining BSP code that I had added for SBC platforms.

Link: https://lore.kernel.org/r/20210824174209.GB160508@windriver.com