arm64: dts: ls1028a: fix watchdog device node

ls1028a platform uses sp805 watchdog, and use 1/16 platform clock as
timer clock, this patch fix device tree node.

Signed-off-by: Chuanhua Han <chuanhua.han@nxp.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>