RFC mfd sharing support

This patchset sets up support for sharing of MFD cells across multiple drivers. This is something we'd like for OLPC power management, but plenty of other drivers do similar things. Every driver seems to have its own way of accomplishing the task, and rather than go that route, I'd rather have a common way to do it in mfd-core.

The parts of this patch series are somewhat separate, so that one can pick and choose. The first group (patches 1-16) makes mfd cells always available (and adjusts any drivers that are using platform_data to use driver_data instead) to mfd clients. The second group (patch 17) adds optional wrappers for the mfd_cell's enable/disable hooks to handle reference counting.

The third group (patch 18) adds support for shared platform_devices, and the last one (patch 19) adapts the cs5535-mfd/olpc-xo1 drivers to make use of the sharing code. These last two groups still need a bit of work, but I was hoping to get feedback on the whole set.