IB/ipath: Allow more flexible user register alignments

User registers have different alignments on different chips (4KB on
older, 64KB on 7220).  Allow mapping the user registers on kernels with
page sizes up to 64K.

Signed-off-by: Ralph Campbell <ralph.campbell@qlogic.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>