mfd: core: Reorder fields in 'struct mfd_cell' to save some memory

Group some variables based on their sizes to reduce hole and avoid padding.
On x86_64, this shrinks the size from 144 to 128 bytes.

As an example:

$ size drivers/mfd/as3722.o (Before)
   text	   data	    bss	    dec	    hex	filename
   9441	    680	     16	  10137	   2799	drivers/mfd/as3722.o

$ size drivers/mfd/as3722.o (After)
   text	   data	    bss	    dec	    hex	filename
   9345	    680	     16	  10041	   2739	drivers/mfd/as3722.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/bb631974888dfe1af593b6280cf30fb913d2d1a4.1676365116.git.christophe.jaillet@wanadoo.fr