powerpc/pseries: Define HVPIPE specific macros

Define HVPIPE specific macros which are needed to support
ibm,send-hvpipe-msg and ibm,receive-hvpipe-msg RTAS calls
and used to handle HVPIPE message events.

Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Tested-by: Shashank MS <shashank.gowda@in.ibm.com>
Reviewed-by: Mahesh Salgaonkar <mahesh@linux.ibm.com>
Reviewed-by: Tyrel Datwyler <tyreld@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250909084402.1488456-3-haren@linux.ibm.com