watchdog: stmp3xxx_rtc_wdt: fix broken email address

My Pengutronix address is not valid anymore, redirect people to the Pengutronix
kernel team.

Reported-by: Harald Geyer <harald@ccbib.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>
Acked-by: Robert Schwebel <r.schwebel@pengutronix.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>