powerpc: retire sbc8548 board support

The support was for this was mainlined 13 years ago, in v2.6.25
[0e0fffe8] just around the ppc --> powerpc migration.

I believe the board was introduced a year or two before that, so it
is roughly a 15 year old platform - with the CPU speed and memory size
that was 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>