add support for LGT8F cpu

[jayzakk]

Add support for the LGT8F-series of CPUs, which behave different from original Atmega due to 4 byte SPI buffer.
Before, SPI does not work, because DC changes level in the middle of a byte - the macro did not know of the SPI buffer and instantly returned instead of waiting for the full sent byte.

For minimal changes, just to make it work, I only changed the AVR_WRITESPI define.
I did not implement the 4 byte buffering (which would make the 16-bit-writes a little bit faster)

As a side note, I also changed the default SPI frequency to 16MHz (at 32MHz CPU speed) for this CPU.

Background:
The LGT8F-Series is a cheaper Atmega328D/P "clone" with more features and faster core, running up to 32MHz.
It's not unusual to get Arduinos with this chip if you buy some Nano or Pro.