Add TI CDCE925 I2C controlled clock synthesizer driver

This driver supports the TI CDCE925 programmable clock synthesizer.
The chip contains two PLLs with spread-spectrum clocking support and
five output dividers. The driver only supports the following setup,
and uses a fixed setting for the output muxes:
  Y1 is derived from the input clock
  Y2 and Y3 derive from PLL1
  Y4 and Y5 derive from PLL2
Given a target output frequency, the driver will set the PLL and
divider to best approximate the desired output.

Signed-off-by: Mike Looijmans <mike.looijmans@topic.nl>
Signed-off-by: Michael Turquette <mturquette@linaro.org>