powerpc/85xx: Add workaround for erratum CPU-A003999

Erratum A-003999: Running Floating Point instructions requires special
initialization.

Impact:
Floating point arithmetic operations may result in an incorrect value.

Workaround:
Perform a read modify write to set bit 7 to a 1 in SPR 977 before
executing any floating point arithmetic operation. This bit can be set
when setting MSR[FP], and can be cleared when clearing MSR[FP].
Alternatively, the bit can be set once at boot time, and never cleared.
There will be no performance degradation due to setting this bit.

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