target/openrisc: Fix madd

Note that the specification for lf.madd.s is confused.  It's
the only mention of supposed FPMADDHI/FPMADDLO special registers.
On the other hand, or1ksim implements a somewhat normal non-fused
multiply and add.  Mirror that.

Reviewed-by: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Signed-off-by: Richard Henderson <rth@twiddle.net>