Documentation: arm: add Marvell Armada 1500 Mini Plus to SoC list

Add known information about the Marvell BG2CDP SoC that's used in
the Google Chromecast 2015 and Kinoma HD devices.

Signed-off-by: Thomas Hebb <tommyhebb@gmail.com>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>