target/arm: Move 'x' and 'a' bit definitions into vmlaldav formats

target/arm: Move 'x' and 'a' bit definitions into vmlaldav formats

All the users of the vmlaldav formats have an 'x bit in bit 12 and an
'a' bit in bit 5; move these to the format rather than specifying them
in each insn pattern.

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