plat/st: stm32mp1: introduce setup_uart_console

Add a new function setup_uart_console for UART console
initialization using device tree information to simplify
sp_min_early_platform_setup().

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: I455b43b105a804590297572a977bc242a526df17
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/180871


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>