x86: tangier: acpi: add I2C6 node

Now that we have I2C#6 working, it's time to add a corresponsing
ACPI binding.

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>