dts: bindings: ipm: Add dummy mbox-cells property

The MBOX driver interface expects a device tree property '#mbox-cells'
which is not known by the IPM driver. This causes build problems when
both drivers are given for a single shared DT node.

To fix this problem for this driver specifically, add a dummy
'#mbox-cells' property to the bindings of the STM32 HSEM IPM driver.
This does not affect any other IPM driver, and the STM32 HSEM IPM driver
is still functioning with this dummy property.

Signed-off-by: Celina Sophie Kalus <hello@celinakalus.de>