dt-bindings: serial: 8250_omap: Add compatible for J721E UART controller

J721e uses a UART controller that is compatible with AM654 UART.
Introduce a specific compatible to help handle the differences if
necessary.

Cc: Sekhar Nori <nsekhar@ti.com>
Cc: Vignesh R <vigneshr@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Tero Kristo <t-kristo@ti.com>

Documentation/devicetree/bindings/serial/omap_serial.txt

 OMAP UART controller
 
 Required properties:
+- compatible : should be "ti,j721e-uart", "ti,am654-uart" for J721E controllers
 - compatible : should be "ti,am654-uart" for AM654 controllers
 - compatible : should be "ti,omap2-uart" for OMAP2 controllers
 - compatible : should be "ti,omap3-uart" for OMAP3 controllers