FYI, I activated the SERIRQ in file
src/soc/intel/fsp_baytrail/southcluster.c by defining SETUPSERIQ and
CONFIG_SERIRQ_CONTINUOUS_MODE in my Kconfig in my mainboard folder.
I can confirm that all the LPC stuffs are working well. (Accesses + IRQs)
Good news! Thanks for the feedback benoit.
I wonder if some or both of the above configuration items should be
build-time options in Kconfig.