ARM: dts: stm32: Add timer interrupts on stm32mp15

The timer units in the stm32mp15x CPUs have interrupts, depending on the
timer flavour either one "global" or four dedicated ones. Add the irqs
to the timer units on stm32mp15x.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>