IB/mlx5: Mmap the HCA's core clock register to user-space

In order to read the HCA's current cycles register, we need
to map it to user-space. Add support to map this register
via mmap command.

Signed-off-by: Matan Barak <matanb@mellanox.com>
Reviewed-by: Moshe Lazer <moshel@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>