watchdog: Convert rdc321x_wdt to use southbridge pci device

The RDC321x MFD southbridge driver will pass a reference to the
southbridge PCI device which should be used by the watchdog driver for its
operations.  This patch converts the watchdog driver to use the pci_dev
pointer and make use of the base register resource which is passed along
with the platform device.

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Acked-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>