Microsoft has released a support statement about running a System Center Configuration Manager 2012 SP1 and System Center Configuration Manager 2012 R2 on a SQL Server 2014 database – but there's a few caveats. Usually, when a support statement like this is released, Microsoft grants blanket supportability. But that's not the case here.
The first, most important caveat is that while SQL Server 2014 is supported as a site database, it must be an upgrade, not a new install. To use SQL Server 2014 as a site database provider, the site had to have been already running, and upgraded from SQL Server 2012, SQL Server 2008, or SQL Server 2008 R2. New ConfigMgr 2012 installations using SQL Server 2014 as the base is not supported.
One other caveat, important to a lot of people, is that WSUS will not work if installed on a Windows 2008 (R2 SP1 and SP2) server with SQL Server 2014 installed on it.
There are a couple other gotchas involved, too, including required updates and hotfixes required to run SQL Server 2014 with ConfigMgr 2012, so make sure you read through the official support announcement on Microsoft's System Center Configuration Manager Team Blog: