watchdog: cadence_wdt: make of_device_ids const.

of_device_ids are not supposed to change at runtime. All functions
working with of_device_ids provided by <linux/of.h> work with const
of_device_ids. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   1962	    612	      4	   2578	    a12	drivers/watchdog/cadence_wdt.o

File size after constify cdns_wdt_of_match:
   text	   data	    bss	    dec	    hex	filename
   2378	    196	      4	   2578	    a12	drivers/watchdog/cadence_wdt.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>