modem_cellular: Add support for the simcom a76xx modem

Add support for the simcom a76xx modem which is similar to the simcom 7080
but has a few key differences. Tested with a simcom A7672SA module but as
there is a single simcom A76XX AT commands manual, the driver should work
with other modems of the series.

Signed-off-by: Olivier Lalonde <o@syskall.com>