ARM: dts: NSP: disable i2c DT entry by default

The i2c device tree entry should be disabled by default to match the
current convention in other device tree files.  Similarily, enable it on
the XMC board, where it is being used.

Signed-off-by: Jon Mason <jon.mason@broadcom.com>
Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>