tools/power turbostat: Fix PMT mmap size and bounds check

Remove assert(offset == 0), to allow non-zero PMT offsets.

Fix the pmt_mmio_open() mapping to round-up (offset + size).

Fix the offset bounds check to disallow running past the end of the region.

[lenb: commit message re-write]
Signed-off-by: Kaushlendra Kumar <kaushlendra.kumar@intel.com>
Signed-off-by: Len Brown <len.brown@intel.com>