powerpc/powermac: Add PowerMac10,2 machine descriptor

This adds support for the Mac Mini's that were quietly rolled out
in 2005. Work still needs to be done to support suspend and
WakeOnLan.

Signed-off-by: Mark Crichton <crichton@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>