mfd: stw481x: Make three arrays static const, reduces object code size

Don't populate the arrays vcore_val, vpll_val and vaux_val on the
stack, instead make them static const.  Makes the object code smaller
by over 370 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
   6971	   3248	     64	  10283	   282b	drivers/mfd/stw481x.o

After:
   text	   data	    bss	    dec	    hex	filename
   6338	   3504	     64	   9906	   26b2	drivers/mfd/stw481x.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>