Nicla Vision: initial variant support

[bogdanarduino]

Initial support for the Nicla Vision. Tested thus far:

• Compilation and flashing of core;
• Compilation of basic blink and serial print sketches;
• LED pins config;
• USB CDC port able to produce output;
• generated zephyr.dts checked against the hardware config for pins and muxes;
• no detailed peripheral checks done at this point.