viafb: add auxiliary device management infrastructure

This patch adds the basic infrastructure and a few stub drivers for
devices that are connected via I2C busses. The infrastructure will
be used to replace and extend the support that is scattered
throughout viafb. The stub drivers are not very useful yet but they
show how the infrastructure works, provide information about the
chips integrated into a system and maybe gather some testers as it
would be very difficult for a single person to get a sane test
environment.
The only thing this actually does is probing the I2C busses which
will hopefully not cause any regressions.

Signed-off-by: Florian Tobias Schandinat <FlorianSchandinat@gmx.de>