modem: cmux: Decouple modem cmux and cellular driver

The modem modules cmux module is currently directly coupled to the
presence of specific modems, rather than being selected by drivers
for whatever hardware wants to request the default MTU of 127 bytes.

This commit the makes the device drivers (for now, modem_cellular)
select the symbol, thus decoupling the modem modules from the
presence of any specific device.

Signed-off-by: Bjarki Arge Andreasen <bjarki.andreasen@nordicsemi.no>