doc: ti: Add readme for AM62x SK

Add info of boot flow and build steps for AM62x SK.

Signed-off-by: Vignesh Raghavendra <vigneshr@ti.com>
Reviewed-by: Bryan Brattlof <bb@ti.com>
diff --git a/doc/board/ti/index.rst b/doc/board/ti/index.rst
index 014a097..250d924 100644
--- a/doc/board/ti/index.rst
+++ b/doc/board/ti/index.rst
@@ -8,3 +8,4 @@
 
    am335x_evm
    j721e_evm
+   am62x_sk