powerpc/opal: Assign numbers to OPAL_MSG macros of enum opal_msg_type

This patch assigns numbers to OPAL_MSG macros of enum opal_msg_type
to prevent accidental insertion of any new value in between and thus
break OPAL API. This is also helpful while backporting mainline kernel
changes to distros which run downlevel kernel and thus don't have all
OPAL messages defined, avoiding unnecessary bugs due to enum values
order mismatch.

Signed-off-by: Vipin K Parashar <vipin@linux.vnet.ibm.com>
Acked-by: Stewart Smith <stewart@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>