gpio-async: convert to ywf

[ppannuto]

Compile-tested only, fairly straight conversion.

There's a mild deviation from the style guide with the wait_for_generic_command instead of 6 different wait_for_[set|clear|...] methods. It mirrors how the driver itself is implemented, and I think is a cleaner design and easier to follow than lots of redundant syscall methods.

More generally, I suspect as we port more of these we may find it makes sense to rethink the guidance around ywf methods, but I'm not a that level of confident with just this one.