plat: allwinner: common: use r_wdog instead of wdog

Some Allwinner H6 has a broken watchdog that doesn't
make the soc reboot.

Use the R_WATCHDOG instead.

Signed-off-by: Clément Péron <peron.clem@gmail.com>
Change-Id: Ie95cc30a80ed517b60b30d6bc2e655a1b53f18ba