Support the LTE modem

[jekhor]

The YB1-X91L contains the LTE modem connected to the SSIC XHCI port. There is no driver for it in the mainline kernel.

See:

• https://github.com/deadman96385/android_kernel_lenovo_yeti/tree/master/drivers/staging/modem_control
• https://wiki.archlinux.org/index.php/Xmm7360-pci
• https://github.com/xmm7360/xmm7360-pci