powerpc/pseries: Detect secure and trusted boot state of the system.

The device-tree properties to check secure and trusted boot state are
different for guests (pseries) compared to baremetal (powernv).

This patch updates the existing is_ppc_secureboot_enabled() and
is_ppc_trustedboot_enabled() functions to add support for pseries.

For pseries the secureboot and trustedboot state are exposed via
device-tree properties /ibm,secure-boot and /ibm,trusted-boot.

The values of ibm,secure-boot under pseries are interpreted as:

  0   - Disabled
  1   - Enabled in Log-only mode. This patch interprets this value as
        disabled, since audit mode is currently not supported for
	Linux.
  2   - Enabled and enforced.
  3-9 - Enabled and enforcing; requirements are at the discretion of
        the operating system.

The values of ibm,trusted-boot under pseries are interpreted as:
  0 - Disabled
  1 - Enabled

Signed-off-by: Nayna Jain <nayna@linux.ibm.com>
Reviewed-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Mimi Zohar <zohar@linux.ibm.com>
[mpe: Drop machdep.h inclusion, tweak change log slightly]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1594813921-12425-1-git-send-email-nayna@linux.ibm.com