watchdog: ath79_wdt: get register base from platform device's resources

The ath79_wdt driver uses a fixed memory address
currently. Although this is working with each
currently supported SoCs, but this may change
in the future. Additionally, the driver includes
platform specific header files in order to be
able to get the memory base of the watchdog
device.

The patch adds a memory resource to the platform
device, and converts the driver to get the base
address of the watchdog device from that.

Signed-off-by: Gabor Juhos <juhosg@openwrt.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>