OMAP4: clock-common: Move the usb dppl configuration to new func

OMAP4: clock-common: Move the usb dppl configuration to new func

usb dpll configuration is done only part of non-essential
dppl configuration however if CONFIG_USB_EHCI_OMAP is defined
we may have to configure usb dpll's for proper functioning
of usb modules. So move the usb dppl configuration to a new func.
and utilise the same during essential dpll configuration.

Signed-off-by: Govindraj.R <govindraj.raja@ti.com>
Tested-by: Stefano Babic <sbabic@denx.de>