mfd sharing support (v2, part 1)

Hi,

Here's my second attempt[0] of the MFD sharing. Rather than go through with the whole thing, this is just the part which makes mfd_cells always available to MFD clients. I'm planning to later remove driver_data, and just use a (renamed) platform_data as a general cleanup, but for now.. Here's the first part.

Again, the goal here is to allow MFD clients to share MFD driver resources. Each platform device that an MFD driver creates has the mfd_cell that created it located in its platform_data area. For drivers that were using the mfd_cell's platform_data field for other purposes, this is now available by grabbing the mfd_cell and looking at its platform_data field.

[0] first attempt -
http://thread.gmane.org/gmane.linux.kernel/1095524/focus=1095546