md.texi: Update documentation of MIPS constraints.

	* doc/md.texi: Update documentation of MIPS constraints.
	* config/mips/constraints.md: New file.
	* config/mips/mips.md: Include it.
	* config/mips/mips.c (mips_char_to_class): Delete.
	(override_options): Don't initialize mips_char_to_class.
	* config/mips/mips.h (mips_char_to_class): Delete.
	(REG_CLASS_FROM_LETTER): Delete.
	(CONST_OK_FOR_LETTER_P): Delete.
	(CONST_DOUBLE_OK_FOR_LETTER_P): Delete.
	(EXTRA_CONSTRAINT_Y): Delete.
	(EXTRA_CONSTRAINT_STR): Delete.
	(CONSTRAINT_LEN): Delete.
	(EXTRA_MEMORY_CONSTRAINT): Delete.

From-SVN: r111868