The MRS instruction should set mode and interrupt bits in the read value
so it is simpler to use a new simulation routine (simulate_mrs) rather
than some modified emulation.

prep_emulate_rd12 is now unused and removed.
Signed-off-by: Jon Medhurst <tixy@yxit.co.uk>
Signed-off-by: Nicolas Pitre <nicolas.pitre@linaro.org>