rockchip: dts: rk3399-puma: update usbhub_enable regulator

rockchip: dts: rk3399-puma: update usbhub_enable regulator

To correctly model the usbhub_enable regulator for U-Boot, we need
to change the settings to:
 * the GPIO polarity is GPIO_ACTIVE_LOW
 * should be set to inactive (enable-active-low) when boot-on settings
   are applied
 * it can be changed at runtime (i.e. remove the always-on)

Signed-off-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Tested-by: Klaus Goger <klaus.goger@theobroma-systems.com>