target-arm: A64: add support for bitfield insns

This patch implements the C3.4.2 Bitfield instructions:
SBFM, BFM, UBFM.

Signed-off-by: Claudio Fontana <claudio.fontana@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <rth@twiddle.net>