powerpc/ieee1275: Enter lockdown based on /ibm, secure-boot

powerpc/ieee1275: Enter lockdown based on /ibm, secure-boot

Read secure boot mode from 'ibm,secure-boot' property and if the secure boot
mode is set to 2 (enforce), enter lockdown. Else it is considered as disabled.
There are three secure boot modes. They are

0 - disabled
     No signature verification is performed. This is the default.
1 - audit
     Signature verification is performed and if signature verification fails,
     display the errors and allow the boot to continue.
2 - enforce
     Lockdown the GRUB. Signature verification is performed and if signature
     verification fails, display the errors and stop the boot.

Now, only support disabled and enforce.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Signed-off-by: Sudhakar Kuppusamy <sudhakar@linux.ibm.com>
Reviewed-by: Stefan Berger <stefanb@linux.ibm.com>
Reviewed-by: Avnish Chouhan <avnish@linux.ibm.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>