powerpc: drop MPC834x_MDS platform support

This 2006 era Modular Development System (MDS) has, at its core
component, a full length card with a PCI-64 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.

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, like
our WR SBC8349 [since retired in v4.18 (2017, commit 3bc6cf5a)]

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 platform.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Acked-by: Li Yang <leoyang.li@nxp.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230220115913.25811-2-paul.gortmaker@windriver.com