serial: 8250_uniphier: remove unused "fifo-size" property

The FIFO size of the UART devices is 64 on almost all UniPhier SoCs
with the exception Pro4TV SoC (MN2WS0235), which used 128 FIFO size.
However, Pro4TV SoC was never upstreamed, and out of production.

So, this property has never been used in a useful way.
Let's remove old unused code.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

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

@@ -6,9 +6,6 @@ Required properties:
 - interrupts: a single interrupt specifier.
 - clocks: phandle to the input clock.
 
-Optional properties:
-- fifo-size: the RX/TX FIFO size.  Defaults to 64 if not specified.
-
 Example:
 	aliases {
 		serial0 = &serial0;
@@ -19,5 +16,4 @@ Example:
 		reg = <0x54006800 0x40>;
 		interrupts = <0 33 4>;
 		clocks = <&uart_clk>;
-		fifo-size = <64>;
 	};