mlx4: Map core_clock page to user space only when allowed

Currently when we map the hca_core_clock page to the user space,
there are vulnerable registers, one of which is semaphore, on
this page as well. If user read the wrong offset, it can modify the
above semaphore and hang the device.

Hence, mapping the hca_core_clock page to the user space only when
user required it specifically.

After this patch, mlx4 core_clock won't be mapped to user space by
default. Oppose to current state, where mlx4 core_clock is always mapped
to user space.

MFC after: 1 week
Reviewed by: kib
Sponsored by: Mellanox Technologies // NVIDIA Networking