[RFC] Add MMC support to barebox

[Juergen Beisert]

This patch is based on some work Sascha made already. I tried to bring in some
light into the MCI framework routines as they are based on code from another
project. Bear with me, if I didn't add useful comments all over the code. I
just wasn't able to understand what happen... If someone has more knowledge
about MCI cards, please take a look at the FIXME/TODOs and make me understand
why the code does what it does.

There is no regular user, yet. The first platform (S3C2440 based) I did the
tests on, is not part of barebox (and will not) and the other platfrom (i.MX23
ChumbOne) is not yet part of barebox. But will coming soon.

Regards,
Juergen