powerpc: retire sbc8641d board support

powerpc: retire sbc8641d board support

The support was for this was added to mainline over 12 years ago, in
v2.6.26 [856cea3130d7] just around the ppc --> powerpc migration.

I believe the board was introduced shortly after the sbc8548 board,
making it roughly a 14 year old platform - with the CPU speed and
memory size typical for that era.

I haven't had one of these boards for several years, and availability
was discontinued several years before that.

Given that, there is no point in adding a burden to testing coverage
that builds all possible defconfigs, so it makes sense to remove it.

Of course it will remain in the git history forever, for anyone who
happens to find a functional board and wants to tinker with it.

Acked-by: Scott Wood <oss@buserror.net>
Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>