Set default I2C clock speed to 100kHz for split_common (#6161)

* Set default I2C clock rate for split_common boards to 100kHz

Default from I2C driver is 400kHz.

* Update documentation for setting I2C clock speed

docs/config_options.md

@@ -93,8 +93,8 @@ This is a C header file that is one of the first things included, and will persi
   * key combination that allows the use of magic commands (useful for debugging)
 * `#define USB_MAX_POWER_CONSUMPTION`
   * sets the maximum power (in mA) over USB for the device (default: 500)
-* `#define SCL_CLOCK 100000L`
+* `#define F_SCL 100000L`
-  * sets the SCL_CLOCK speed for split keyboards. The default is `100000L` but some boards can be set to `400000L`.
+  * sets the I2C clock rate speed for keyboards using I2C. The default is `400000L`, except for keyboards using `split_common`, where the default is `100000L`.
 
 ## Features That Can Be Disabled
 

quantum/split_common/post_config.h

@@ -4,6 +4,10 @@
     #define RGBLIGHT_SPLIT
   #endif
 
+  #ifndef F_SCL
+    #define F_SCL 100000UL  // SCL frequency
+  #endif
+
 #else  // use serial
   // When using serial, the user must define RGBLIGHT_SPLIT explicitly
   //  in config.h as needed.