p9Layouts: Need to mark read-only partitions with read-only flag

[anoo1]

In the P9 layouts like the defaultPnorLayout_64.xml, the read-only flag is not being used to makr the read-only partitions. The read-only flag is needed by the BMC mailbox code to know if the partition should be written to.
The layout files already use the preserved flag to mark some partitions like NVRAM, so read-only would just be a different value from the same flag category. Reference: https://github.com/open-power/hostboot/blob/master/src/usr/pnor/common/ffs_hb.H

[williamspatrick]

@wghoffa - No one has looked at this since July. This is going to start to become a problem for the OpenBMC code.

[bofferdn]

Removing Bill as assignee