iio: adc: at91: make array startup_lookup static

Don't populate array startup_lookup on the stack but instead make
it static.  Makes the object code smaller.

Before:
   text	   data	    bss	    dec	    hex	filename
  21754	   5440	    128	  27322	   6aba	drivers/iio/adc/at91_adc.o

After:
   text	   data	    bss	    dec	    hex	filename
  21613	   5496	    128	  27237	   6a65	drivers/iio/adc/at91_adc.o

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>