usb: phy: qmp: Make clk_set_rate on pipe_clk optional

On MSM8994, the QMP PHY pipe_clk does not support clk_set_rate(),
as the underlying mechanism to switch its source between XO and
the output of the QMP pipe clock is unnecessary. Add a device tree
property to specify that it is not needed. In that case
clk_prepare_enable() should be called only after the PHY
initialization is complete.

Change-Id: I42cf7fb456c05e7bf2b46e86cd66fd92cdf2bd51
Signed-off-by: Jack Pham <jackp@codeaurora.org>