According to the Errata ERR004346 (WDOG: WDOG SRS bit requires to be written
twice), add another two writes to ensure there must be at least two
writes happen in the same one 32kHz clock period.
Without this two additional writes, the system will not reset always.

Tested with i.MX6Quad and i.MX6 UltraLite

Signed-off-by: Christian Hemp <c.hemp@phytec.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>