armv8: ls1028a: configure PMU's PCTBENR to enable WDT

The SP805-WDT module on LS1028A requires configuration of PMU's
PCTBENR register to enable watchdog counter decrement and reset
signal generation. The watchdog clock needs to be enabled first.

Signed-off-by: Thomas Schaefer <thomas.schaefer@kontron.com>
Signed-off-by: Zhao Qiang <qiang.zhao@nxp.com>
Reviewed-by: Priyanka Jain <priyanka.jain@nxp.com>