dts: nordic: introduce wdt0 / wdt1 distinction

Add wdt0 node labels for all watchdogs. Keep the existing wdt node
labels in place for backwards compatibility.

Add a wdt1 node to the nRF5340 application core .dtsi, but leave it
disabled. Leave the wdt node's label set to "WDT" on this core for
backwards compatibility also, in case any users are getting it with a
string literal instead of asking the devicetree. This can be changed
later after this round of watchdog changes has been shaken out and
tested more.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>