mfd: Attaching a node to new 'struct mfd_cell' of_compatible variable

Applying a succinct description to the of_compatible variable recently
added to the mfd_cell struct. Also link to the documentation page where
more information can be found about compatible properties.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>