dt-bindings: clk: Extend binding doc for Stingray SOC

Update iproc clock dt-binding documentation with
Stingray pll and clock details.

Signed-off-by: Sandeep Tripathy <sandeep.tripathy@broadcom.com>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>