powerpc/pseries/htmdump: Add documentation for H_HTM debugfs interface

Documentation for HTM (Hardware Trace Macro) debugfs interface
and how it can be used to configure/control the HTM operations.

Signed-off-by: Athira Rajeev <atrajeev@linux.ibm.com>
Tested-by: Venkat Rao Bagalkote <venkat88@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250420180844.53128-10-atrajeev@linux.ibm.com