AVR: Remove "Atmel" from header comment.

gcc/
	* config/avr/avr.h: Remove "Atmel" from header comment.
	* config/avr/avr.cc: Same.
	* config/avr/avr.md: Same.
	* config/avr/avr.opt: Same.
	* config/avr/avr-dimode.md: Same.
	* config/avr/avr-fixed.md: Same.
	* config/avr/constraints.md: Same.
	* config/avr/predicates.md: Same.
	* config/avr/avr-log.cc: Same.
	* config/avr/avrlibc.h: Same.
	* config/avr/specs.h: Same.
	* common/config/avr/avr-common.cc: Same.
	* doc/install.texi: Same.
	* config/avr/avr-arch.h: Adjust header comment.
	* config/avr/avr-c.cc: Same.
	* config/avr/avr-mcus.def: Same.
	* config/avr/avr-modes.def: Same.
	* config/avr/avr-passes.cc: Same.
	* config/avr/avr-passes.def: Same.
	* config/avr/avr-protos.h: Same.
	* config/avr/driver-avr.cc: Same.
	* config/avr/elf.h: Same.
	* config/avr/gen-avr-mmcu-specs.cc: Same.
	* config/avr/gen-avr-mmcu-texi.cc: Same.