target-arm: provide skeleton for a64 insn decoding

provide a skeleton for a64 instruction decoding in translate-a64.c,
by dividing instructions into the classes defined by the
ARM Architecture Reference Manual(DDI0487A_a) C3

Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>
Signed-off-by: Alex Bennée <alex@bennee.com>
Reviewed-by: Alex Bennée <alex@bennee.com>
Reviewed-by: Richard Henderson <rth@twiddle.net>