target/arm/cpu64: introduce ARM_V8_FP16 feature bit

Signed-off-by: NAlex Bennée <alex.bennee@linaro.org>
Reviewed-by: NRichard Henderson <richard.henderson@linaro.org>
Message-id: 20180227143852.11175-3-alex.bennee@linaro.org
[PMM: postpone actually enabling feature until end of the
 patch series]
Signed-off-by: NPeter Maydell <peter.maydell@linaro.org>

target/arm/cpu.h

@@ -1408,6 +1408,7 @@ enum arm_features {
     ARM_FEATURE_V8_SHA3, /* implements SHA3 part of v8 Crypto Extensions */
     ARM_FEATURE_V8_SM3, /* implements SM3 part of v8 Crypto Extensions */
     ARM_FEATURE_V8_SM4, /* implements SM4 part of v8 Crypto Extensions */
+    ARM_FEATURE_V8_FP16, /* implements v8.2 half-precision float */
 };
 
 static inline int arm_feature(CPUARMState *env, int feature)