GMAC API

[tmplt]

Copied from an internal Gitlab instance. By @martinmortsell.

[martinmortsell]

I put some effort into adding configuration options to the new_gmac() method.
Now mii/rmii, speed, duplex, and mac can be configured by the user.

The tx/rx buffers are still hardcoded in the HAL, I envision an api similar to the heapless::Vec where buffers are defined through const generics and maybe stored in the Gmac struct.

Interrupt support is still missing.

[michalfita]

@martinmortsell this PR is a draft that doesn't pass CI. Any change to get this finished here?

[martinmortsell]

This code definitely needs another pass before merging.
Currently the big issue is that there is no guarantee that the frame buffers are placed statically in memory. As a user one can get around that by declaring the buffers as static. However, that's not really good enough to merge.

Other changes that should be done is clean up the generic variables, and add a feature gate to the smoltcp implementation.

Maybe I'll have time to fiddle with this during the autumn sometime, we'll see.