pinctrl: atlas6: Add I2S external clock input pingroup

The I2S controller can use the external clock as reference clock with
master mode. But based on different hardware or software design, this
external clock might be needed or not needed.
So the external input pin can be an independent pinctrl group, and the
card driver can decice to get it or not.

Signed-off-by: Rongjun Ying <rongjun.ying@csr.com>
Signed-off-by: Barry Song <Baohua.Song@csr.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>