dt-bindings: arm: ti: Add bindings for AM642 SK

commit 0d54e0b48d28a45a6e9ebaa665391ec64f639e96 from
git://git.ti.com/ti-linux-kernel/ti-linux-kernel.git



AM642 StarterKit (SK) board is a low cost, small form factor board
designed for TI’s AM642 SoC.
Add DT binding documentation for AM642 SK.

Signed-off-by: Lokesh Vutla <lokeshvutla@ti.com>
Signed-off-by: Xulin Sun <xulin.sun@windriver.com>