drivers: modem: optional factory reset at modem boot

Some modems, under some conditions, have a tendency to get stuck without
a connection due to cached state. We have observed this on some Simcom
LTE modems after large cellular outages. The modems are unable to escape
their cached state for some reason unless they're factory reset (or a
cache clearence is forced in some other way).

This commit allows for the modem to be factory reset at each boot. This
minimizes dependencies on external state by ensuring each power-up is as
similar as possible.

Signed-off-by: Benjamin Lindqvist <benjamin.lindqvist@endian.se>