powerpc: 512x: Rename wdt@ node to watchdog@

The watchdog.yaml schema prescribes a node name of "timer" or "watchdog"
rather than the abbreviation "wdt".

Signed-off-by: J. Neuschäfer <[e-mail]>
Reviewed-by: Christophe Leroy <[e-mail]>
Signed-off-by: Madhavan Srinivasan <[e-mail]>
Link: https://patch.msgid.link/[e-mail]

arch/powerpc/boot/dts/mpc5121.dtsi

···
 		};
 
 		/* Watchdog timer */
-		wdt@900 {
+		watchdog@900 {
 			compatible = "fsl,mpc5121-wdt";
 			reg = <0x900 0x100>;
 		};