dt-bindings: Add DT bindings info for FlexRM ring manager

This patch adds device tree bindings document for the FlexRM
ring manager found on Broadcom iProc SoCs.

Acked-by: Rob Herring <robh@kernel.org>
Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Anup Patel <anup.patel@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>