powerpc: drop MPC8272_ADS platform support

The MPC8272-ADS also supported other 82xx CPU variants, had 64MB RAM,
8MB flash, and like the 85xx ADS platforms, was on a fairly large PCB
in order to have space for breakout connectors for all the features.

These 82xx platforms are two decades old, and originally made for a
small group of industry related people in order to assist in new OEM
board designs.  Given that, it makes sense to remove support today.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230224204959.17425-2-paul.gortmaker@windriver.com