board: Move beagleplay under beagle vendor folder

Move beagleplay support away from ti/am62x to it's own beagle vendor
folder.

This forms the starting point for new beagle platforms added under it's
own board vendor folder.

As part of this create all the associated files with a bare minimum
beagleplay.c file.

Suggested-by: Andrew Davis <afd@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Bryan Brattlof <bb@ti.com>