mfd: omap-usb-host: Use clock names as per function for reference clocks

Use a meaningful name for the reference clocks so that it indicates the
function.

Update the OMAP4+ USB Host node as well to be in sync with the changes.

Acked-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Roger Quadros <rogerq@ti.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>