pru: Transform patterns to use hard register constraints

Refactor patterns to use hard register constraints.  With this, all the
single-register classes currently defined for PRU can be removed.

gcc/ChangeLog:

	* config/pru/constraints.md (MULDST_REGS): Remove constraint.
	(MULSRC0_REGS): Ditto.
	(MULSRC1_REGS): Ditto.
	* config/pru/predicates.md (pru_muldst_operand): Compare
	register number directly.
	(pru_mulsrc0_operand): Ditto.
	(pru_mulsrc1_operand): Ditto.
	* config/pru/pru.h (enum reg_class): Remove single-reg classes.
	(REG_CLASS_NAMES): Ditto.
	(REGNO_REG_CLASS): Ditto.
	* config/pru/pru.md (mulsi3): Switch to hard register constraints.
	(umulsidi3): Ditto.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>