clk: iproc: define Broadcom iProc clock binding

Document the device tree binding for Broadcom iProc architecture based
clock controller

Signed-off-by: Ray Jui <rjui@broadcom.com>
Reviewed-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Michael Turquette <mturquette@baylibre.com>