Add an I2C interface

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Identify an appropriate I2C interface chip and add a JST connector compatible with STEMMA and QWIIC. This will allow many I2C sensors to be added to the system without requiring additional memory-mapped I/O.

Try to find a single chip solution for interfacing to the I2C bus; ideally it should perform parallel/serial conversion to avoid the need to bit-bang the interface from the 6805.

Open questions:

• [ ] What interface chip is to be used?
• [ ] Through-hole or surface mount?
• [ ] Is interrupt support needed or is polling sufficient?
• [ ] Should the I2C interface consume existing PIO pins or is a dedicated memory-mapped I/O needed?

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This blog post might be relevant: http://retro.hansotten.nl/6502-sbc/lee-davison-web-site/i2c-bus-interface/

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I2C controller: https://www.nxp.com/docs/en/data-sheet/PCF8584.pdf

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AN95068: C routines for the PCx8584