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>

Prepare 0.3.4

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

Update CHANGELOG

Add support for attiny13a

Update CHANGELOG

Use the same timer patch for atmega1280 as for atmega2560

Prepare 0.3.3

Update CHANGELOG

Add support for `asm!()`

Update CHANGELOG

Add support for ATmega1284p

Update CHANGELOG

Add ATmega128RFA1 support

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

Update CHANGELOG

Add support for ATtiny1614

Prepare 0.3.2

Update CHANGELOG

Return the previous interrupt status upon disabling interrupts

This commit addresses #88 by returning a boolean which reflects the
previous state of the GIE flag upon disabling interrupts. This allows
`critical-section` to implement itself for AVR, while not breaking the
existing `avr-device` API.

Add support for ATtiny816

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