media: imx7-mipi-csis: make array 'registers' static const, makes object smaller

Don't populate the array 'registers' on the stack but instead make it
static const. Makes the object code smaller by 10 bytes.

Before:
   text	   data	    bss	    dec	    hex	filename
  20138	   5196	    128	  25462	   6376	staging/media/imx/imx7-mipi-csis.o

After:
   text	   data	    bss	    dec	    hex	filename
  20032	   5292	    128	  25452	   636c	staging/media/imx/imx7-mipi-csis.o

(gcc version 9.2.1, amd64)

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>