board: phytec: phycore_am68x: Turn off pmic watchdog

board: phytec: phycore_am68x: Turn off pmic watchdog

Starting with pmic firmware version 5 the watchdog is enabled with a
timeout window of 13 minutes by default.

The TI EVMs use a dip switch to control a gpio on the pmic which gets
configured as DISABLE_WDOG by the firmware.
On the phycore_am68x there is no such dip switch so pull resistors would be
necessary. Using mcu_i2c0 to turn off the pmic watchdog works with all
hardware and pmic firmware combinations.

Signed-off-by: Dominik Haller <d.haller@phytec.de>