powerpc/pseries/htmdump: Add htm start support to htmdump module

Support starting of Hardware Trace Macro (HTM) function
via debugfs interface. Under debugfs folder
"/sys/kernel/debug/powerpc/htmdump", add file "htmstart".
The interface allows starting of htm via this file by
writing value "1". Also allows stopping of htm tracing by
writing value "0" to this file. Any other value returns
-EINVAL.

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/20250420180844.53128-4-atrajeev@linux.ibm.com