watchdog: Fix build failure with OF changes

commit 61c7a080 ( of: Always use
'struct device.of_node' to get device node pointer.) missed
drivers/watchdog/mpc8xxx_wdt.c. This patch fixes it

Signed-off-by: Michael Guntsche <mike@it-loops.com>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>