dts: stm32h7: define mailbox controller based on stm32 hsem

The ipm_stm32_hsem driver is a virtual mailbox based on STM32 HSEM.
Since we already have LL_HSEM(low-level HSEM) API on the hal_stm32
module, looks there is no need to implement a new HSEM driver.
But there are still have some limitations, the HSEMx interrupt is
not shareable. If another HSEM user wants to use the HSEMx interrupt,
the ipm_stm32_hsem mailbox needs to be disabled.

Signed-off-by: HaiLong Yang <hailong.yang@brainco.cn>