Add support for ATtiny404

Small fixes

Remove all llvm_asm macros

Add basic documentation for beginners

Add support for ATmega164pa

Adds the ATDF file provided from Microchip, and makes changes analog to
290613454fbdc5e4ac98e53deccaf74dafc88963.
This has been tested on real hardware.

Signed-off-by: Tronje Krabbe <tkrabbe@jusst.de>
Signed-off-by: Oskar Munz <omunz@jusst.de>

Only build docs for some MCUs on docs.rs

Sadly we are now starting to hit the limits of docs.rs with the amount
of supported MCUs.  Only build documentation for a subset there and
mention that docs for the rest can be built locally.

Ref: #98

Add support for attiny13a

Add support for `asm!()`

Add support for ATmega1284p

Add ATmega128RFA1 support

Adds basic ATmega128RFA1 support. Adjusts the common USART patches for
UCSZ? slightly to work with the ATmega128RFA1's ATDF.

Add support for ATtiny1614

Add support for ATtiny816

Add basic support for the ATtiny816 of the tinyAVR1-series.

Add support for ATtiny2313/A

Add ATtiny167 support

Add ATtiny202 support

All of the svd patches are in patch/common/attiny-0-series.yaml which should
work for all of the 0-series (but may only work for ATtiny20* and ATtiny40*,
  but not 80* or 160* given how the datasheet is split)

Add AT90USB1286 support

Ref: #80

Add ATmega8U2

Improve error message when no device is selected

Introduce a new `device-selected` feature to simpilify the logic for
showing an error when no device was selected.

Add attiny84 and atmega644 to `src/lib.rs`

Add attiny841 and attiny861