clk: add DT fixed-clock binding support

Add support for DT "fixed-clock" binding to the common fixed rate clock
support.

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
[Rob Herring] Rework and move into common clock infrastructure
Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>