devicetree: bindings: IB: Add binding document for Hisilicon RoCE

This patch added DTS binding document for Hisilicon RoCE driver.

Signed-off-by: Wei Hu <xavier.huwei@huawei.com>
Signed-off-by: Lijun Ou <oulijun@huawei.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>