MIPS/GAS: Correct `-O0' and `-O' option help, add `-O1' and `-O2'

MIPS/GAS: Correct `-O0' and `-O' option help, add `-O1' and `-O2'

Match commit 4ffff32f75b1 ("Match mips_optimize to the -O option
supplied") and adjust `--help' output for `-O0', `-O', `-O1' and `-O2'
options.

	gas/
	* config/tc-mips.c (md_show_usage): Correct help text for `-O0'
	and `-O'.  Mention `-O1'.  Add `-O2' and its description.