plat/st: Add STM32MP_UART_PROGRAMMER

Add a compilation flag to support of STM32CubeProgrammer on UART.

Signed-off-by: Patrick Delaunay <patrick.delaunay@st.com>
Change-Id: I8a6988bddaf9aad6f297c865439c7daae0e64205
Reviewed-on: https://gerrit.st.com/c/mpu/oe/st/tf-a/+/180865


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>