target/arm: Implement MVE VCVT between floating and fixed point

target/arm: Implement MVE VCVT between floating and fixed point

Implement the MVE VCVT insns which convert between floating and fixed
point.  As with the Neon equivalents, these use essentially the same
constant encoding as right-shift-by-immediate.

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