watchdog: i6300esb: support multiple devices

Support multiple i6300esb devices simultaneously, by removing the single
device restriction in the original design and leveraging the multiple
device support of the watchdog subsystem.

This patch replaces the global definitions of watchdog device data with
a dynamically allocated structure. This structure is allocated during
device probe, so multiple independent structures can be allocated if
multiple devices are probed.

Signed-off-by: Radu Rendec <rrendec@arista.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>