MIPS: VZ: Only include loongson_regs.h for CPU_LOONGSON64

Only Loongson64 platform has and needs loongson_regs.h, including it
unconditionally will cause build errors.

Fixes: 7f2a83f1 ("KVM: MIPS: Add CPUCFG emulation for Loongson-3")
Cc: stable@vger.kernel.org
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Huacai Chen <chenhc@lemote.com>
Message-Id: <1596891052-24052-1-git-send-email-chenhc@lemote.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>