powerpc/pseries: Add a char driver for physical-attestation RTAS

powerpc/pseries: Add a char driver for physical-attestation RTAS

The RTAS call ibm,physical-attestation is used to retrieve
information about the trusted boot state of the firmware and
hypervisor on the system, and also Trusted Platform Modules (TPM)
data if the system is TCG 2.0 compliant.

This RTAS interface expects the caller to define different command
structs such as RetrieveTPMLog, RetrievePlatformCertificat and etc,
in a work area with a maximum size of 4K bytes and the response
buffer will be returned in the same work area.

The current implementation of this RTAS function is in the user
space but allocation of the work area is restricted with the system
lockdown. So this patch implements this RTAS function in the kernel
and expose to the user space with open/ioctl/read interfaces.

PAPR (2.13+ 21.3 ibm,physical-attestation) defines RTAS function:
- Pass the command struct to obtain the response buffer for the
  specific command.
- This RTAS function is sequence RTAS call and has to issue RTAS
  call multiple times to get the complete response buffer (max 64K).
  The hypervisor expects the first RTAS call with the sequence 1 and
  the subsequent calls with the sequence number returned from the
  previous calls.

Expose these interfaces to user space with a
/dev/papr-physical-attestation character device using the following
programming model:

 int devfd = open("/dev/papr-physical-attestation");
 int fd = ioctl(devfd, PAPR_PHY_ATTEST_IOC_HANDLE,
                      struct papr_phy_attest_io_block);
 - The user space defines the command struct and requests the
   response for any command.
 - Obtain the complete response buffer and returned the buffer as
   blob to the command specific FD.
 size = read(fd, buf, len);
 - Can retrieve the response buffer once or multiple times until the
   end of BLOB buffer.

Implemented this new kernel ABI support in librtas library for
system lockdown

Signed-off-by: Haren Myneni <haren@linux.ibm.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250416225743.596462-8-haren@linux.ibm.com