target/m68k: add 680x0 "move to SR" instruction

Some cleanup, and allows SR to be moved from any addressing mode.
Previous code was wrong for coldfire: coldfire also allows to
use addressing mode to set SR/CCR. It only supports Data register
to get SR/CCR (move from)

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20180104012913.30763-15-laurent@vivier.eu>