pinctrl: merrifield: Add I²S bus 2 pins to groups and functions

It is useful to control I²S bus 2 pins if we would like to connect
an audio codec.

Reported-by: mouse <xllacyx@gmail.com>
Reported-by: Pierre-Louis Bossart <pierre-louis.bossart@linux.intel.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Mika Westerberg <mika.westerberg@linux.intel.com>