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
1 file changed