Suppress severe warnings

These warnings are reported as severe by ISE 14.7, however none of them appear to be real issues - in all cases the HDL compiler is assuming that adding `1` to a signal will cause it to overflow. This may or not be true. For some reason, the compiler can be silenced by instead adding `1b1`, ie a signal 1  that is of length 1.

I have checked that there is no real error being reported by the warnings, so I believe it is safe to suppress them.