i2c: core: add api to provide frequency mode strings

Some I2C drivers like Designware and HiSilicon will print the
bus frequency mode information, so add a public one that everyone
can make use of.

Tested-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Jarkko Nikula <jarkko.nikula@linux.intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Yicong Yang <yangyicong@hisilicon.com>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

include/linux/i2c.h

@@ -51,6 +51,9 @@ struct module;
 struct property_entry;
 
 #if IS_ENABLED(CONFIG_I2C)
+/* Return the Frequency mode string based on the bus frequency */
+const char *i2c_freq_mode_string(u32 bus_freq_hz);
+
 /*
  * The master routines are the ones normally used to transmit data to devices
  * on a bus (or read from them). Apart from two basic transfer functions to