Add support for MCP23S17

[zybeon]

I don't think it would be to difficult to add support for the SPI GPIO expander right? Or is this feature already in there?

[substring]

That's something we thought of, is it really worth compared to the I2C MCP23O17 ?

[zybeon]

It is required when using a DPI screen with anything other than the compute modules. The only ports left when using RGB565 16-bit (Mode3) is SPI, 3 GPIO, and 2 PWM. (GPIO:9,10,11,18,19,25,26,27)

[zybeon]

Guess a little more involved. Using this should really help; https://github.com/piface/libmcp23s17

And example use: https://github.com/piface/libmcp23s17/blob/master/example.c