usb: qusb: Select QUSB2 clk source based on phy clk scheme

In order to lock QUSB2 PHY PLL QUSB2PHY_PLL_TEST register needs
to be updated to indicate clk source based on the
TCSR PHY_SCHEME_SEL register value. If PHY_CLK_SCHEME_SEL bit is
set single ended clk scheme is selected otherwise differential
clk scheme is selected.

Change-Id: I946fe364c857d45cf3f3cacabd5788b826cad9f0
Signed-off-by: Hemant Kumar <hemantk@codeaurora.org>