powerpc/cell: Remove support for IBM Cell Blades

IBM Cell Blades used the Cell processor and the "blade" server form
factor. They were sold as models QS20, QS21 & QS22 from roughly 2006 to
2012 [1]. They were used in a few supercomputers (eg. Roadrunner) that
have since been dismantled, and were not that widely used otherwise.

Until recently I still had a working QS22, which meant I was able to
keep the platform support working, but unfortunately that machine has
now died.

I'm not aware of any users. If there is a user that wants to keep the
upstream support working, we can look at bringing some of the code back
as appropriate.

See previous discussion at [2].

Remove the top-level config symbol PPC_IBM_CELL_BLADE, and then
the dependent symbols PPC_CELL_NATIVE, PPC_CELL_COMMON, CBE_RAS,
PPC_IBM_CELL_RESETBUTTON, PPC_IBM_CELL_POWERBUTTON, CBE_THERM, and
AXON_MSI. Then remove the associated C files and headers, and trim
unused header content (some is shared with PS3).

Note that PPC_CELL_COMMON sounds like it would build code shared with
PS3, but it does not. It's a relic from when code was shared between the
Blade support and QPACE support.

Most of the primary authors already have CREDITS entries, with the
exception of Christian, so add one for him.

[1]: https://www.theregister.com/2011/06/28/ibm_kills_qs22_blade
[2]: https://lore.kernel.org/linuxppc-dev/60581044-df82-40ad-b94c-56468007a93e@app.fastmail.com

Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jeremy Kerr <jk@ozlabs.org>
Acked-by: Segher Boessenkool <segher@kernel.crashing.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20241218105523.416573-1-mpe@ellerman.id.au

CREDITS

@@ -2197,6 +2197,9 @@ S: Markham, Ontario
 S: L3R 8B2
 S: Canada
 
+N: Christian Krafft
+D: PowerPC Cell support
+
 N: Maxim Krasnyansky
 E: maxk@qualcomm.com
 W: http://vtun.sf.net