MIPS: use mips_isa enum instead hardcoded numbers

MIPS: use mips_isa enum instead hardcoded numbers

Currently mips-cpu.defs, mips.h, netbsd.h and config.gcc are
using hardcoded numbers for isa level.

Let's replace them with more readable enum mips_isa.

gcc/ChangeLog:
	* config/mips/mips.h (struct mips_cpu_info): define enum mips_isa;
	  use enum instead of int for 'isa' member.
	* config.gcc, config/mips/mips.c, config/mips/mips-cpus.def,
	  config/mips/netbsd.h: replace hardcoded numbers with enum.