refactor(stm32mp15-fdts): remove unused PMIC nodes

The onkey and watchdog features of the PMIC are not used in TF-A for
STM32MP15 boards. Remove the nodes from DT.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I2933e0bdc5843fcb549a817742106d9c66097869

fdts/stm32mp157c-ed1.dts

@@ -176,17 +176,6 @@
 				regulator-active-discharge = <1>;
 			};
 		};
-
-		onkey {
-			compatible = "st,stpmic1-onkey";
-			power-off-time-sec = <10>;
-			status = "okay";
-		};
-
-		watchdog {
-			compatible = "st,stpmic1-wdt";
-			status = "disabled";
-		};
 	};
 };