Update:

> Marvell never had "strictly ARM cores" to offer. All their ARM cores to date have
> been own implementations of some licensed ARM ISA version, not actual cores
> licensed from ARM Ltd.

Chip with Cortex-A9 core announced in February 2011 (back then still hiding that fact by only touting it as "ARMv7", making everybody believe it would have Marvell's own Sheeva PJ4 core):
http://www.marvell.com/company/news/pressDetail.do?releaseID=1519
http://www.marvell.com/communication-processors/pxa978/

Chips with Cortex-M3 core announced in January 2012:
http://www.marvell.com/company/news/pressDetail.do?releaseID=2003
http://www.marvell.com/company/news/pressDetail.do?releaseID=2001
http://www.marvell.com/wireless/88MZ100/
http://www.marvell.com/green-technology/smart-energy/88MC200/

Chips with Cortex-A9 cores announced in August 2012:
http://www.marvell.com/company/news/pressDetail.do?releaseID=2616
http://www.marvell.com/communication-processors/pxa986/
http://www.marvell.com/communication-processors/pxa988/

Analysts say that the move to using ARM Ltd's Cortex-A9 core over their own Sheeva PJ4B core is due to pleasing Marvell's Android customers, as more and more Android software is using ARM Ltd's NEON SIMD whereas Marvell's own cores implement WMMX SIMD, which is not compatible with NEON.
In contrast to that explanation, Marvell does offer SoCs which couple their Sheeva PJ4B core with NEON:

https://morph.zone/modules/newbb_plus/viewtopic.php?topic_id=7675&forum=3&start=211