reset: zx2967: constify zx2967_reset_ops.

File size before:
   text	   data	    bss	    dec	    hex	filename
    794	    232	      0	   1026	    402	drivers/reset/reset-zx2967.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
    842	    184	      0	   1026	    402	drivers/reset/reset-zx2967.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>