watchdog: armada_37xx_wdt: use do_div for u64 division

When the driver is built on 32 bit architectures during compile test,
the linker complains about "__udivdi3" being undefined. We have to use
do_div macro instead of the division operator when dividing u64 value.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@linux-watchdog.org>