mips/kvm: Support FPU in MIPS KVM guests

Support the new KVM_CAP_MIPS_FPU capability, which allows the host's FPU
to be exposed to the KVM guest.

The capability is enabled if the guest core has an FPU according to its
Config1 register. Various config bits are now writeable so that KVM is
aware of the configuration (Config1.FP) and so that QEMU can
save/restore the guest modifiable bits (Config5.FRE, Config5.UFR,
Config5.UFE). The FCSR/FIR registers and the floating point registers
are now saved/restored (depending on the FR mode bit).

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Leon Alrae <leon.alrae@imgtec.com>
Cc: Aurelien Jarno <aurelien@aurel32.net>
Signed-off-by: Leon Alrae <leon.alrae@imgtec.com>
