CBUSx GPIO support

[chrysn]

For use with the mikroe USB click adapter, it would be quite convenient to have GPIO support for the FT2232H BC pin family: as far as I understand, its BCBUS[1..4] pins should be usable as GPIO pins -- at least, that's how they are used on this particular board. Then, ftdi-embedded-hal could be directly used to develop embedded drivers from the comfort of a Linux environment.

If you don't get around to implementing it, I'd also appreciate any useful hints for when I try to implement this myself.

By the way: Thanks for providing this crate. It solves a long-standing dilemma I've been facing, which I can now resolve by just writing applications on embedded-hal, and run it on this crate, on an equivalent crate for other similar chips, or on kernel peripherals through linux-embedded-hal, as long as I'm not writing a kernel driver (which I am not) -- it implements the "Using a library to abstract away kernel-native or FTDI GPIO/SPI" route.

[chrysn]

With #51, port BCBUS3 can be accessed like this:

    let device_b = ftdi::find_by_vid_pid(0x0403, 0x6010)
        .interface(ftdi::Interface::B)
        .open()
        .unwrap();
    let hal_b = hal::FtHal::init_default(device_b).unwrap();
    let mut pin = hal_b.c3().unwrap();