avr: sphinx: port gen-avr-mmcu to RST

gcc/ChangeLog:

	* Makefile.in: Add vpath default for .rst files.
	* config/avr/avr-devices.cc: For port RST.
	* config/avr/avr-mcus.def: Update path.
	* config/avr/gen-avr-mmcu-texi.cc: Moved to...
	* config/avr/gen-avr-mmcu-rst.cc: ...here.
	* config/avr/t-avr: Update to rst.
	* doc/gcc/gcc-command-options/machine-dependent-options/avr-mmcu.rst: Re-generate.