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

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: Nishanth Menon <nm@ti.com>
Tested-by: Kishon Vijay Abraham I <kishon@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210226184251.26451-2-lokeshvutla@ti.com