overlays: sc16is752-spi1: Add xtal parameter

The other sc16is75x overlays have an xtal parameter to allow a
different crystal frequency to be specified, but sc16is752-spi1
doesn't. Fix this omission.

See: https://www.raspberrypi.org/forums/viewtopic.php?f=107&t=273234



Signed-off-by: Phil Elwell <phil@raspberrypi.com>

arch/arm/boot/dts/overlays/README

@@ -2114,6 +2114,7 @@ Info:   Overlay for the NXP SC16IS752 Dual UART with SPI Interface
 
 Load:   dtoverlay=sc16is752-spi1,<param>=<val>
 Params: int_pin                 GPIO used for IRQ (default 24)
+        xtal                    On-board crystal frequency (default 14745600)
 
 
 Name:   sdhost

arch/arm/boot/dts/overlays/sc16is752-spi1-overlay.dts

@@ -56,6 +56,7 @@
 	};
 
     __overrides__ {
-      int_pin = <&sc16is752>,"interrupts:0";
+		int_pin = <&sc16is752>,"interrupts:0";
+		xtal = <&sc16is752_clk>,"clock-frequency:0";
     };
 };