The ondemand governor (devfreq_ondemand.c) references dev->governor_data
to store its private state, and defines devfreq_ondemand(), but neither
the field in struct devfreq_s nor the function declaration were present
in include/nuttx/devfreq.h. As a result, building with
CONFIG_DEVFREQ_GOV_ONDEMAND=y failed to compile.
These two definitions were originally introduced by a downstream commit
that was not part of the devfreq upstreaming series, so the gap only
surfaced when the ondemand governor is enabled (which additionally
requires !CONFIG_SCHED_CPULOAD_NONE and is off by default).
Add the governor_data field to struct devfreq_s and declare
devfreq_ondemand() alongside the other governors.
Signed-off-by: guanyi <guanyi@xiaomi.com>