ARM: dts: atlas6: add a separate pingroup for i2s mclk output

The I2S controller can output mclk to external audio codec. But by
hardware design, some codecs need mclk and some codecs do not need
mclk. So the mclk pin can be an independent pinctrl group, and the
card driver can get it or not based on boards.

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