Power sim900 shield with raspberry pi
You can wire the single power supply to the shield as well as to 5V pin on the GPIO connector of the RaspPI.
Some PI's have a fuse protection. In order to not circumvent this protection, an even better suggestion is to use a mini USB, cut it, isolate the 5V and GND wires and connect those to the  single power supply.

Better not to use the mini USB to supply the RaspPI and wire the 5V pin to the shield, as the (unknown?) current drawn by the shield runs over the RaspPI. This might cause noise maybe too much current run through the RaspPI PCB traces.


  The SIM900 chip partially requires currents of up to 2A, which the Arduino cannot afford.


The RaspPI neither can afford this.