powerpc: remove retired sbc834x support

I no longer have a functional version of this board for even the most
basic sanity boot testing, and they have not been available for purchase
for quite some years now.

There is no point in adding a burden to testing coverage that does
walk all the possible defconfigs, so with all the above in mind, it
makes sense to remove it.  Of course it will remain in the git history
for anyone who happens to stumble on one and wants to tinker with it.

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>