powerpc: drop MPC836x_MDS platform support

This 2006 era Modular Development System (MDS) has, at its core component,
a full length card with a PCI edge.  No case.  Serial and network
connectors were on card, so it could optionally be fitted with plastic
stand-offs and run stand-alone off a power brick.

This is very similar to the MPC834x_MDS removed in the prior commit, but
with this board variant as an evolutionary step.  DDR2 was now an option,
and the card edge was revised down to PCI-32 as PCI-64 never got traction.
But overall the form factor and design goals were unchanged.

Like all the MDS systems, it was meant as a vehicle to get the CPU out
early to hardware OEMs so software and board development could take place
in parallel.

To that end, the BGA CPU was held in place with a mechanical spring loaded
pressure assembly (vs. solder) so that early rev silicon could be replaced
in the field.  Not for COTS deployment!

These were made in limited numbers and availability preference was given
to partners who were planning to make their own boards.

Given that the whole reason for existence was to assist in enabling new
board designs [not happening for 10+ years], and that they weren't
generally available, and that the hardware wasn't really hobbyist friendly
even for retro computing, it makes sense to retire the support for this
particular platform.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Li Yang <leoyang.li@nxp.com>
[mpe: Drop stale reference to MPC836x_MDS in arch/powerpc/boot/Makefile]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230220115913.25811-3-paul.gortmaker@windriver.com