scripts/genpinctrl: Add generation of additional functions

For non-F1 series:
Add generation of additional functions by inferring mode based
on AF detection instead of the mode value filed in config file.
This allows to detect that a signal should use 'analog' mode
when no AF function is associated to it.

Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>