serial: stm32: add earlycon support

Add early console support in stm32 uart driver.

Signed-off-by: Alexandre Torgue <alexandre.torgue@foss.st.com>
Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20220419085330.1178925-4-valentin.caron@foss.st.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>