target/arm: Revise decoding for disas_add_sub_imm

target/arm: Revise decoding for disas_add_sub_imm

The current Arm ARM has adjusted the official decode of
"Add/subtract (immediate)" so that the shift field is only bit 22,
and bit 23 is part of the op1 field of the parent category
"Data processing - immediate".

Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Message-id: 20200626033144.790098-11-richard.henderson@linaro.org
Suggested-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>