USB: ice40-hcd: Add XO clock support

The bridge chip require a 19.2MHz clock for USB operations.
It is supplied by the PMIC on MSM8916. Add the XO clock.
The XO clock can operate either in pin-control or direct
mode. When it operates in pin-control mode, the clk-en gpio
also needs to be asserted to enable the clock.

Change-Id: Id181fd5edfe22c57f3c958e081454b4fe987a097
Signed-off-by: Pavankumar Kondeti <pkondeti@codeaurora.org>