clk: hi3798cv200: add support for HISTB_USB2_OTG_UTMI_CLK

The clock HISTB_USB2_OTG_UTMI_CLK is defined by device tree bindings in
include/dt-bindings/clock/histb-clock.h, but hasn't been supported by
hi3798cv200 clock driver.  Let's add the support for it.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>