IB/ehca: Support large page MRs

Add support for MR pages larger than 4K on eHCA2. This reduces
firmware memory consumption.  If enabled via the mr_largepage module
parameter, the MR page size will be determined based on the MR length
and the hardware capabilities -- if the MR is >= 16M, 16M pages are
used, for example.

Signed-off-by: Joachim Fenkes <fenkes@de.ibm.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>