Documentation: DT: omap_serial: document missing compatible

The compatible "ti,am4372-uart" is used in arch/arm/boot/dts/am4372.dtsi
but not documented. Add necessary documentation.
Signed-off-by: Sekhar Nori <nsekhar@ti.com>
Acked-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Peter Hurley <peter@hurleysoftware.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/serial/omap_serial.txt

@@ -4,6 +4,7 @@ Required properties:
 - 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
+- compatible : should be "ti,am4372-uart" for AM437x controllers
 - reg : address and length of the register space
 - interrupts or interrupts-extended : Should contain the uart interrupt
                                       specifier or both the interrupt