Support for GPIO pins on external chips.

[marcuschangarm]

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.

[ciarmcom]

ARM Internal Ref: IOTSFW-1302