Possible Solution
The most simple way to find this is a simple text search through the *.dts file for the pin name. You may be able to look at some random pin in the *.dts file and compare it to the schematic to see the naming scheme to give you an idea of what it should look like.
For example, if the the schematic has a GPIO Pin3 on Port A, you might see something like "gpio_portA-3" or some other permutation. Look for a pin in the *.dts file that also appears in the schematic to identify the naming pattern.
Once you have that, search for the pin you're actually interested in. If you don't find it right away, try different variations, and perhaps try regular expressions to match varying separating characters. If you find it, you should be able to see what the parent node is, which will tell you what the pin does.
Given this poor guy's adventures with reverse-engineering the BeagleBone Black's pinmux mappings, you should consider yourself lucky if a simple text search actually works for you.
Caveat
Unfortunately, there's no guarantee that the pin you're interested in will even be present in the FDT. If a pin doesn't require direct interaction from the OS to work properly, it won't be there. Consider this example from devicetree.org:
/ {
#address-cells = <1>;
#size-cells = <1>;
...
serial@101f0000 {
compatible = "arm,pl011";
reg = <0x101f0000 0x1000 >;
};
serial@101f2000 {
compatible = "arm,pl011";
reg = <0x101f2000 0x1000 >;
};
gpio@101f3000 {
compatible = "arm,pl061";
reg = <0x101f3000 0x1000
0x101f4000 0x0010>;
};
interrupt-controller@10140000 {
compatible = "arm,pl190";
reg = <0x10140000 0x1000 >;
};
spi@10115000 {
compatible = "arm,pl022";
reg = <0x10115000 0x1000 >;
};
...
};
This block describes a couple of memory-mapped devices, including serial, SPI, and interrupt controllers, and even GPIOs. All of those, with the possible exception of the interrupt controller, must use pins, yet they aren't specifically present here. This is because the manipulation of the pins is intended to be done by hardware, and the OS shouldn't have to know or care which pins are used. All the OS needs to know is how to interact with the controllers' registers, so the pin information may be left out of the FDT.
