irqchip/stm32: Don't clear rising/falling config registers at init

Falling and rising configuration and status registers are not banked.
As they are shared with M4 co-processor, they should not be cleared
at probe time, else M4 co-processor configuration will be lost.

Fixes: f9fc1745 ("irqchip/stm32: Add host and driver data structures")
Signed-off-by: Loic Pallardy <loic.pallardy@st.com>
Signed-off-by: Fabien Dessenne <fabien.dessenne@st.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>