This article was posted on the Operations Manager Engineering blog this week detailing the newly included support for SCOM 2012 R2 having SQL Server 2014 Enterprise, Standard (64-bit) as its database.

They note the following issues and workarounds to be aware of:

Issue: Installation to a SQL 2014 cluster using Cluster Shared Volumes (CSV) is blocked by SCOM 2012 setup.
Workaround: Deploy to a physical server and then use the documentation here to move the database to the CSV.

Issue: Upgrade of SQL SRS is blocked due to SCOM 2012 use of Custom Security Extensions.
Workaround: Uninstall SCOM Reporting and run ResetSRS.exe to clear the custom security extensions. Once that is complete, upgrade SQL and install SCOM 2012 reporting again. Can find similar steps in the documentation here.