usb: phy: nop: Manage PHY clock

If the PHY has a clock associated to it then manage the clock.
We just enable the clock in .init() and disable it in .shutdown().

Add clk_rate parameter in platform data and configure the
clock rate during probe if supplied.

Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Felipe Balbi <balbi@ti.com>