drivers/modem: Add generic cellular modem driver

The added cellular modem driver is a naive driver, which
shall serve as a template for implementing tailored
drivers for modems like the UBLOX-R4. It uses only
generic at commands, described in 3gpp, and protocols,
like CMUX and PPP.

A binding for the BG95 has been added, which replaces
the quectel,bg9x. This is neccesary since the BG95 does
not have a usable reset pin, the reset and powerkey are
internally connected to each other.

Signed-off-by: Bjarki Arge Andreasen <baa@trackunit.com>