soc: st: stm32: hsem: update description for fifth HSEM

The fifth HSEM (#define is equal to 4 due to zero-indexing) is used on
STM32H7 to synchronize the two cores. Update the comment above the SEMID
define to reflect this alternate usage. Also remove the associated define
CFG_HW_ENTRY_STOP_MODE_MASK_SEMID, which is unused.

Signed-off-by: Mathieu Choplain <mathieu.choplain@st.com>