RDMA/erdma: Add main include file

Add ERDMA driver main header file, defining internal used data structures
and operations. The defined data structures includes *cmdq*, which is used
as the communication channel between ERDMA driver and hardware.

Link: https://lore.kernel.org/r/20220727014927.76564-4-chengyou@linux.alibaba.com


Signed-off-by: Cheng Xu <chengyou@linux.alibaba.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>