dt-bindings: serial: 8250_omap: Add compatible for AM654 UART controller

AM654 uses a UART controller that is only partially compatible with
existing 8250 UART. UART DMA integration is substantially different
and even a match against standard 8250 or omap4 would result in
non-working UART once DMA is enabled by default.

Introduce a specific compatible to help build up the differences in
follow on patches.

Cc: Sekhar Nori <nsekhar@ti.com>
Cc: Vignesh R <vigneshr@ti.com>
Signed-off-by: Nishanth Menon <nm@ti.com>
Acked-by: Rob Herring <robh@kernel.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/omap_serial.txt

 OMAP UART controller
 
 Required properties:
+- 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
 - compatible : should be "ti,omap4-uart" for OMAP4 controllers