dt: add clock binding doc to primecell bindings

Add clock binding information for primecell peripherals. For most, a
clock input name of "apb_pclk" is required. Any primecell peripherals
which are different will need to be documented separately.

Signed-off-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>