Add AT command to disconnect LPUART

[janakj]

MKRWAN1300 boards have a SPI flash chip connected to the same lines as the LPUART port on the Type ABZ module. This makes it impossible to access the flash while the LPUART port is active. Implement a new AT command that will make it possible to temporarily disconnect Type ABZ LPUART so that the application running on the host MCU can access the flash. The Type ABZ module will be sleeping in the meantime and could be woken up via a GPIO.

Commits implementing this feature are in PR #117.