watchdog: coh901327: make license text and module licence match

Licence text is specifying "GPLv2" but the MODULE_LICENSE is set to "GPLv2
or later".

See include/linux/module.h:
"GPL"				[GNU Public License v2 or later]
"GPL v2"			[GNU Public License v2]

When on it, add SPDX identifier tag.

Signed-off-by: Marcus Folkesson <marcus.folkesson@gmail.com>
Reviewed-by: Philippe Ombredanne <pombredanne@nexb.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>