MIPS: CI20: DTS: Add I2C nodes

Adding missing I2C nodes and some peripheral:
- PMU
- RTC

Signed-off-by: Alexandre GRIVEAUX <agriveaux@deutnet.info>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: robh+dt@kernel.org
Cc: mark.rutland@arm.com
Cc: ralf@linux-mips.org
Cc: jhogan@kernel.org
Cc: linux-mips@vger.kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: devicetree@vger.kernel.org