usb: dwc2: Rename hibernation to partial_power_down

No-op change, only rename.

This code was misnamed originally. It was only responsible for partial
power down and not for hibernation.

Rename core_params->hibernation to core_params->power_down,
dwc2_set_param_hibernation() to dwc2_set_param_power_down().

Signed-off-by: Vardan Mikayelyan <mvardan@synopsys.com>
Signed-off-by: John Youn <johnyoun@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>