powerpc/powernv: Add queue mechanism for early messages

When issuing a BMC soft poweroff during IPL, the poweroff can be lost
so the machine would not poweroff.

This is because opal messages can be received before the opal-power
code registered its notifiers.

Fix it by buffering messages. If we receive a message and do not yet
have a handler for that type, store the message and replay when a
handler for that type is registered.

Signed-off-by: Deb McLemore <debmc@linux.vnet.ibm.com>
[mpe: Single unlock path in opal_message_notifier_register(), tweak
      comments/formatting and change log.]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/1526868278-4204-1-git-send-email-debmc@linux.vnet.ibm.com