Every ATmega board that can run an Real Time Timer can also run
an emulated Real Time Clock.
Got all supported boards by adding `FEATURES_REQUIRED += arch_8bit`
to `tests/periph_rtt` and running
sed -i 's/FEATURES_PROVIDED += periph_rtt/
FEATURES_PROVIDED += periph_rtc\n
FEATURES_PROVIDED += periph_rtt/g'
on them.
- Using @warning Doxygen command to make warning more visible
- Used IEC units for size specifications
- Aligned markdown source of the table
- Added note that the 5V pin is not connected to the board and cannot be used
to power the board
