docs(maintainers): update ST platform ports title

STM32MP1 is no more the only product to be supported in TF-A with the
new STM32MP2. Change "STM32MP1 platform port" to "STMicroelectronics
platform ports" to better reflect this.

Signed-off-by: Yann Gautier <yann.gautier@st.com>
Change-Id: I30b1fd4310d38092e3e815cb635b474fc84bdc30
1 file changed