clk: add highbank clock support

This adds real clock support to Calxeda Highbank SOC using the common
clock infrastructure.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
[mturquette@linaro.org: fixed up invalid writes to const struct member]
Signed-off-by: Mike Turquette <mturquette@linaro.org>