doc: board: Move am62x_beagleplay to it's own vendor

Move BeaglePlay documentation to beagle as a board vendor and update
references accordingly.

Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Bryan Brattlof <bb@ti.com>
diff --git a/doc/board/ti/k3.rst b/doc/board/ti/k3.rst
index 5167925..f3bb664 100644
--- a/doc/board/ti/k3.rst
+++ b/doc/board/ti/k3.rst
@@ -30,8 +30,8 @@
 .. toctree::
    :maxdepth: 1
 
-   am62x_beagleplay
    am62x_sk
+   ../beagle/am62x_beagleplay
    ../toradex/verdin-am62
    am64x_evm
    am65x_evm