drivers: mbox: Add driver for STM32 HSEM

This driver implements a simple MBOX device which supports a single
instance, two channels (one for each direction), and only signalling
mode with no data transfer. Signalling to another core is achieved by
taking and giving two hardware semaphores, similar to the STM32 HSEM
IPM driver.

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