dt-bindings: arm: ti: Add missing compatibles for j721e/j7200 evms

Add compatibles for j721e and j7200 evms to allow for newer platforms
to distinguish themselves.

While doing this, maintain support for older style of description where
the board compatibility was not required.

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Suman Anna <s-anna@ti.com>
Link: https://lore.kernel.org/r/20210925201430.11678-2-nm@ti.com