stm32mp1: move get_uart_address() prototype to common header file

This function should be defined for all ST platforms.

Change-Id: I9b636d3b5e19d31c9b960f5349e1f252cc74d764
Signed-off-by: Yann Gautier <yann.gautier@st.com>
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/150064


Reviewed-by: CITOOLS <smet-aci-reviews@lists.codex.cro.st.com>