imx: imx_clock: usb: Add USB clock API

imx: imx_clock: usb: Add USB clock API

This set of patches adds a very minimal layer of USB enabling patches to
clock.c. Unlike the watchdog or UART blocks the USB clocks pertain to PHYs,
the main USB clock etc, not to different instances of the same IP block.

As a result this patch-set takes the clock CCGR clock identifier directly
rather than as an index of an instance of blocks of the same type.

Signed-off-by: Bryan O'Donoghue <bryan.odonoghue@linaro.org>