MIPS: define_attr perf_ratio in mips.md

The accurate cost of an pattern can get with
	 insn_count * perf_ratio

The default value is set to 0 instead of 1, since that
we will need to distinguish the default value and it is
really set for an pattern.  Since it is not set for most
patterns yet, to use it, we will need to be sure that it's
value is greater than 0.

This attr will be used in `mips_insn_cost`.

gcc

	* config/mips/mips.md (perf_ratio): New attribute.