define Maxmem for ia64, the only platform that didn't have it

This is a direct commit to stable/10 as the platform was removed
in the newer branches.

Maxmem is required for compiling fwohci(4) on ia64 since commit
r310081, MFC of r277511.
It was easier to add Maxmem than to make a special case for ia64
in fwohci.

Reported by: jhb, gjb
Discussed with: kib, jhb