powerpc/mpc85xx: Only emulate the unimplemented FP instructions on corenet64

We have split the math emulation into two parts. This makes it
possible to just emulate the unimplemented floating point instructions
on these boards.

Signed-off-by: Kevin Hao <haokexin@gmail.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>