(enum options): add OPTION_RMW_ISA for -mrmw option.
(struct option md_longopts): Add mrmw option.
(md_show_usage): add -mrmw option description.
(md_parse_option): Update isa details if -mrmw option specified.
* doc/c-avr.texi: Add doc for new option -mrmw.
* gas/avr/avr.exp: Run new tests.
* gas/avr/rmw.d: Add test for additional ISA support.
* gas/avr/rmw.s: Ditto.