dt-bindings: arm: ti: Add compatible for J721E SK

J721E Starter Kit (SK)[1] is a low cost, small form factor board
designed for TI’s J721E SoC. Add j721e-sk into compatible enum.

[1]https://www.ti.com/tool/SK-TDA4VM



Signed-off-by: Sinthu Raja <sinthu.raja@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Nishanth Menon <nm@ti.com>
Link: https://lore.kernel.org/r/20210929081333.26454-2-sinthu.raja@ti.com