plat/st: add function get_uart_address

Add a platform function to get address from UART instance.

Change-Id: Iecf052d219788664843bf1f3120964c92d4d85de
Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/180869


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>
Reviewed-by: Yann GAUTIER <yann.gautier@st.com>
Tested-by: Yann GAUTIER <yann.gautier@st.com>