feat(st-uart): manage STM32MP_RECONFIGURE_CONSOLE

If the flag STM32MP_RECONFIGURE_CONSOLE is set in BL32, the UART init
should be skipped if the UART clock is set to zero. This will be used
when configuring the default console, after an early console has been
configured.

Signed-off-by: Yann Gautier <yann.gautier@foss.st.com>
Change-Id: Icbc640c7bdd6342f9c3ec1586a0d0c64127b18b8