usb: dwc3: Powerdown the SS PHY while connected to HS device

SNPS controller can wakeup both HS PHY and SS PHY (for UTMI clock and
PIPE clock) for any xhci command or doorbell rings. This might result
in additional power consumption when only HS device is connected to
root port. To avoid additional power consumption when operating in
high-speed only mode, power down SS PHY.

Change-Id: I35195934c617d7c581976e34da73777bfa13fe6f
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>