dt-bindings: clock: add description of LPC32xx USB clock controller

NXP LPC32xx USB controller has a subdevice, which controls USB AHB
slave, USB OTG, USB OHCI, USB device and I2C controller to USB phy
clocks, this change adds description of the clock controller, for more
details reference LPC32xx User's Manual, namely USB control, OTG clock
control and OTG clock status registers.

Signed-off-by: Vladimir Zapolskiy <vz@mleia.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>