ARM: dts: add on-board INA231 sensors for exynos5422-odroidxu3

The odroid-xu3 has 4 INA231 current sensors on board which can be
accessed from the Linux via the hwmon interface.

There is one sensor for each of these power rails:

- A15 cluster: VDD_ARM
- A7 cluster: VDD_KFC
- GPU: VDD_G3D
- memory: VDD_MEM

In addition to adding the sensors, LDO26 from the PMIC needs to be
enabled because it's powering these sensor.

Cc: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Cc: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Reviewed-by: Sjoerd Simons <sjoerd.simons@collabora.co.uk>
Signed-off-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Kukjin Kim <kgene@kernel.org>