ARM: shmobile: koelsch dts: Add VDD MPU regulator for DVFS

The CA15 cluster is capable of voltage scaling. Add the regulator
in the i2c6 node, to allow the generic CPUFreq driver to use it.

Enable the i2c6 pin mux and the device node as well since the
da9210 is connected to that bus.

Note: In R-CAR Gen2, each frequency is using the same voltage,
and DVS control is not used. Therefore, this patch set the
voltage(Vmin/Vmax) to 1000mv.

Signed-off-by: Gaku Inami <gaku.inami.xw@bp.renesas.com>
Acked-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>