MAINTAINERS: Add watchdog maintainers entry

I've been handling "inofficially" the watchdog related patches for a few
years now. Let's make this official and add a tree for it and also add
myself here in the MAINTAINERS file.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Tom Rini <trini@konsulko.com>
Cc: Harald Seiler <hws@denx.de>
Reviewed-by: Tom Rini <trini@konsulko.com>