arm64: dts: ti: k3-j7200-common-proc-board: Add j7200-evm compatible

Add j7200-evm compatible to the board to allow the board to distinguish
itself from other platforms that may be added in the future.

Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Suman Anna <s-anna@ti.com>
Link: https://lore.kernel.org/r/20210925201430.11678-5-nm@ti.com