(md_assemble): Handle opcodes with optional operands (lpm,elpm).
(avr_operand): Handle 'a', 'v' and 'z' constraint letters needed
for `fmul', `movw' and `lpm R,Z' instructions.
(avr_operands): Warn if current opcode is a two-word instruction
and previous opcode was cpse/sbic/sbis/sbrc/sbrs.
(avr_opcodes): New commands added.
(REGISTER_P): Check 'a' and 'v' constraint letters.
(mcu_types): New MCU added.
