Create a function to reset the SIM808 module

In case the module freezes or connectivity is disrupted a reset could be useful.
Create a function void system_reset(); that resets the module then the stm32

- STM32 GPIOF PF7 is connected to the Reset pin on the SIM808 module
- Pull down the Reset pin at least 105ms to reset the module
- once the SIM808 is reset with success then reset the MCU.