dts: evb-rk3399: add init voltage node for vdd-center

Add a regulator-init-microvolt for vdd_center regulator
so that we can get a init value for driver probe.
Not like pmic regulator, the PWM regulator do not have a
known default output value, so we would like to init the
regulator when driver probe.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>