watchdog: bcm2835_wdt: constify _ops and _info structures

These are never modified, so might as well be const.

Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>