dt-bindings: mfd: stm32-timers: Document how to specify interrupts

The timer units in the stm32mp1 CPUs have interrupts, depending on the
timer flavour either one "global" or four dedicated ones. Document how
to formalize these in a device tree.

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20220519162838.695404-1-u.kleine-koenig@pengutronix.de