x86: dts: edison: configure I2C#6 pins

Now that we have the pinctrl driver for Merrifield in place we can make
use of it and set I2C#6 pins appropriately.

Initial configuration came from the firmware.  Which quite likely has
been used in the phones, where that is not part of Atom peripheral, is
in use. Thus we need to override the leftover.

Signed-off-by: Georgii Staroselskii <georgii.staroselskii@emlid.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>