mips: enable O32_FP64 and MSA

CONFIG_MIPS_O32_FP64_SUPPORT is required to support MSA for O32.
It requires CPU >= mips32r1. It is OK for us: currently our
baseline is mips32r2/mips64r2.

Malta can use different CPUs, some of them may support MSA.
Loongson 3A/B 4000 will support MSA.
The only CPU currently we support has no MSA is octeon.