ARM: dts: STiH407-family: Add missing pwm irq

Add the missing interrupt node to properly probe
the pwm device.

This fix following error log :
[    0.208119] sti-pwm 9510000.pwm: Failed to obtain IRQ
[    0.222352] pwm-regulator pwm-regulator: Failed to get PWM: -517

Signed-off-by: Lionel Debieve <lionel.debieve@st.com>
Signed-aff-by: Patrice CHOTARD <patrice.chotard@st.com>