dts: bindings: mbox: Add STM32 HSEM MBOX driver

Add a device tree binding for the new driver. Since there already
exists an IPM driver using the unsharable hardware semaphore interrupt,
the new driver is not added to any boards or SOCs per default to avoid
compatibility problems. See #37300 for the IPM driver.

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