usb: phy: Enable sleep clock for USB HS PHY

The Synopsys HS Femto PHY requires sleep clock to
be enabled to trigger PHY interrupts.
Enable the sleep clock so the PHY can trigger interrupts
when in suspend state.

Change-Id: I872d3b5dcc0a7aabc3f91a709039adc909eed8a6
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>