Update assembler documentation on some AVR cores.

Update assembler documentation on some AVR cores.

	PR 21472
	* config/tc-avr.c (mcu_types): Add entries for: attiny212,
	attiny214, attiny412, attiny414, attiny814, attiny1614,
	attiny1616, attiny1617, attiny3214, attiny3216, attiny3217.
	(md_show_usage): Adjust doc for "avrxmega3".
	* doc/c-avr.texi (AVR options) [-mmcu=]: Adjust doc for avrxmega3.
	Add MCUs: attiny212, attiny214, attiny412, attiny414, attiny416,
	attiny417, attiny814, attiny816, attiny817, attiny1614,
	attiny1616, attiny1617, attiny3214, attiny3216, attiny3217.