dt-bindings: serial: Remove obsolete cavium-uart.txt

cavium-uart.txt binding is already covered by 8250.yaml, so remove it.
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20230707221602.1063972-1-robh@kernel.orgSigned-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/serial/cavium-uart.txt

-* Universal Asynchronous Receiver/Transmitter (UART)
-
-- compatible: "cavium,octeon-3860-uart"
-
-  Compatibility with all cn3XXX, cn5XXX and cn6XXX SOCs.
-
-- reg: The base address of the UART register bank.
-
-- interrupts: A single interrupt specifier.
-
-- current-speed: Optional, the current bit rate in bits per second.
-
-Example:
-	uart1: serial@1180000000c00 {
-		compatible = "cavium,octeon-3860-uart","ns16550";
-		reg = <0x11800 0x00000c00 0x0 0x400>;
-		current-speed = <115200>;
-		interrupts = <0 35>;
-	};