target-arm: A64: Add "Floating-point data-processing (2 source)" insns

This patch adds emulation for the "Floating-point data-processing (2 source)"
group of instructions.

Signed-off-by: Alexander Graf <agraf@suse.de>
[WN: Commit message tweak, merge single and double precision patches. Rebase
 and update to new infrastructure. Incorporate FMIN/FMAX support patch by
 Michael Matz.]
Signed-off-by: Will Newton <will.newton@linaro.org>
[PMM:
 * added convenience accessors for FP s and d regs
 * pulled the field decode and opcode validity check up a level]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>
