armv8/ls2080a: configure PMU's PCTBENR to enable WDT

The SP805-WDT module on LS2080A 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: Yunhui Cui <yunhui.cui@nxp.com>
Reviewed-by: York Sun <york.sun@nxp.com>