ARM: dts: bcm2711: Use proper compatible in PM/Watchdog node

A new compatible string was introduced specifically for BCM2711, so make
use of it.

Signed-off-by: Nicolas Saenz Julienne <nsaenzjulienne@suse.de>
Signed-off-by: Stefan Wahren <stefan.wahren@i2se.com>
Reviewed-by: Peter Robinson <pbrobinson@gmail.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>