rockchip: dts: correct vdd_log setting for firefly-rk3399

Add regulator-init-microvolt for driver to init the regulator,
and the min output value is not 800000mV for the PWM2 io domain has
changed to VCC3V0 instead of VCC1V8 in rockchip evb, we need to
correct it with the value measured when PWM2 output HIGH.

Signed-off-by: Kever Yang <kever.yang@rock-chips.com>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>