serial: stm32: extend max number of U(S)ART to 9

STM32MP25x got 9 instances of U(S)ART. So extend STM32_MAX_PORTS to 9, in
order to handle all instances.

Signed-off-by: Valentin Caron <valentin.caron@foss.st.com>
Link: https://lore.kernel.org/r/20240112095300.2004878-3-valentin.caron@foss.st.com


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