mfd: arizona: Make array mclk_name static, shrinks object size

Don't populate the array mclk_name on the stack but instead make it
static. Makes the object code smaller by 23 bytes:

Before:
   text	   data	    bss	    dec	    hex	filename
  38050	  11604	     64	  49718	   c236	linux/drivers/mfd/arizona-core.o

After:
   text	   data	    bss	    dec	    hex	filename
  38027	  11604	     64	  49695	   c21f	linux/drivers/mfd/arizona-core.o

(gcc version 8.2.0 x86_64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Charles Keepax <ckeepax@opensource.cirrus.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>