powerpc/85xx: Enable the math emulation for the corenet64_smp_defconfig

I got the following error on my t4240qds board.
  ntpd[2713]: unhandled signal 4 at 0fd5b448 nip 0fd5b448 lr 0fd5b424 code 30001

The root cause is that the float point instruction 'fsqrt' is used.
But this instruction is not implemented on e6500 core. Even this
does seem a gcc bug, I would like to enable the math emulation
in the kernel to workaround this kind of issue.

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