dt-bindings: mfd: stm32-timers: Remove #address/size cells from required properties

address-cells and size-cells can't be declared as "required" properties
as they are not needed if subnodes don't have a "reg" entry.

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Reviewed-by: Fabrice Gasnier <fabrice.gasnier@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>