[POWERPC] 83xx: Fix Kconfig to only enable FP math emulation for the MPC832x

Updated MATH_EMULATION depends to be on PPC_MPC832x instead of PPC_83xx.  Only
the the MPC832x has no floating point unit in the core.  Updated the other
83xx defconfigs that got math emulation turned on incorrectly.

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>