build(changelog): add stm32mp13 and stm32mp15 scopes

The STM32MP1 series includes STM32MP13 and STM32MP15. As some features
may be dedicated to one SoC variant, add the 2 entries in the scopes
list.
While at it, correct the title for STM32MP1.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I521d0e1dfdda0638ab9970c93821cf08efbd183a