drivers: modem: gsm_ppp: Add support for Quectel modems

Add support for Quectel modem in gsm_ppp modem driver.

The CMUX cmd is based on MUX application notes v1.0(2020-09-22)
for BG95, BG77 and BG600L. Tested and working on EC21e.

As the gsm_ppp doesn't do power control, it is required to power
on the modem manually in the application.

Signed-off-by: Yong Cong Sin <yongcong.sin@gmail.com>