watchdog/aspeed: fix variable type to store reload value

Initially from Anton D. Kachalov" <mouse@yandex-team.ru> but the SoB was
missing.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Acked-by: Andrew Jeffery <andrew@aj.id.au>
Message-id: 20170920064915.30027-1-clg@kaod.org
[clg: change commit log and subject
      replace UL suffix by ULL ]
Signed-off-by: Cédric Le Goater <clg@kaod.org>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>