powerpc: drop MPC8540_ADS and MPC8560_ADS platform support

Based on the revision history in the manual(s), these e500-v1
platforms were first available around 2002.

Like a lot of evaluation boards, they attempted to provide break-out
connectors for all possible features, and that combined with four
PCI-X slots (and the age/era) meant for a considerably large board.

As I recall it, from a Linux point of view, the biggest difference
between 8540 and 8560 was in the UART implementation, and that is
reflected in a diff of the defconfigs.

In any case, these are over 20 years old, and by today's standards
only have a small amount of DDR1 memory, and were not widely available.

Given that, it makes sense to remove support from them in 2023.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230620043300.197546-2-paul.gortmaker@windriver.com