NXP provides a tool, called "Pins", that is used to configure the processors pin functions. This can be a very useful tool for implementing a custom peripheral set for your application.
To get started, download the Pins application here:
http://www.nxp.com/products/software-and-tools/software-development-tools/processor-expert-and-embedded-components/software-suites/pins-tool-for-i.mx-application-processors:PINS-TOOL-IMX
Also, download the standard phyCORE-i.MX7 SoM configuration here:
https://develop.phytec.com/download/attachments/34113565/phyCORE-i.MX7_1458_1.mex?api=v2
Open the "phyCORE-i.MX7_1458_1.mex" file using Pins (see below). This shows the standard configuration for the SoM:

If you need to customize the SoM pinout, the Pins tool is a great tool to figure out what is possible. Keep in mind that this tool is for the processor and not every processor pin is available on the SoM connector. With that in mind, note that the "Label" column in the picture below indicates the processor pins location on the SoM connector. For example, UART1_RXD is configured on processor pin T1, which is connected to the SoM connector at X1 B15. Likewise, UART1_TXD is configured on processor pin V1, which is connected to the SoM connector at X1 B16.
