DBC is currently accessed only from the platform access mailbox and
a lot of that implementation's communication path is intertwined
with DBC. Add an abstraction layer for pointers into the mailbox.

No intended functional changes.
Signed-off-by: Mario Limonciello <mario.limonciello@amd.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>