mfd: Add refcounting support to mfd_cells

This provides convenience functions for sharing of cells across
multiple mfd clients.  Mfd drivers can provide enable/disable hooks
to actually tweak the hardware, and clients can call
mfd_shared_cell_{en,dis}able without having to worry about whether
or not another client happens to have enabled or disabled the
cell/hardware.

Note that this is purely optional; drivers can continue to use
the mfd_cell's enable/disable hooks for their own purposes, if
desired.

Signed-off-by: Andres Salomon <dilinger@queued.net>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>