MIPS: KVM: Limit Trap-and-Emulate to MIPS32R2 only

After tons of fixes to get Trap-and-Emulate build on Loongson64,
I've got panic on host machine when trying to run a VM.

I found that it can never work on 64bit systems. Revewing the
code, it looks like R6 can't supportrd by TE as well.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Message-Id: <20200710063047.154611-3-jiaxun.yang@flygoat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>