arm64: dts: ti: k3-j721e-common-proc-board: Add j721e-evm compatible

Add j721e-evm compatible to the board to allow the board to distinguish
itself from other platforms that are pending to be added.

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Suman Anna <s-anna@ti.com>
Link: https://lore.kernel.org/r/20210925201430.11678-4-nm@ti.com