arm/translate-a64: initial decode for simd_three_reg_same_fp16

arm/translate-a64: initial decode for simd_three_reg_same_fp16

This is the initial decode skeleton for the Advanced SIMD three same
instruction group.

The fprintf is purely to aid debugging as the additional instructions
are added. It will be removed once the group is complete.

Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20180227143852.11175-9-alex.bennee@linaro.org
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>