Support for GPIO pins on external chips.

I have this I/O expander chip that runs over I2C.

In "the other OS" I could make each pin look and behave like any other GPIO pin on the main MCU which meant I could pass it along to other modules like the SPI driver.

It would be useful to be able to do the same in mbed OS, for example by making the methods in Digital*/InterruptIn virtual or by adding support in gpio.c/InterruptIn.cpp.

Start by reading the Digital*/InterruptIn interfaces and the gpio.c and InterruptIn.cpp entry points mentioned in the issue. Trace how GPIO objects are passed to modules such as the SPI driver, then determine what support is needed for pins on an I2C I/O expander. Done means external-chip pins can behave like ordinary GPIO pins and be passed to other modules.

Written by the indexing model from the issue text.