Pull mailbox updates from Jassi Brar:
- Minor improvement : avoid requiring unnecessary startup/shutdown
callback that many drivers seem to not need
- New controller driver for Qualcomm's APCS IPC
* 'mailbox-for-next' of git://git.linaro.org/landing-teams/working/fujitsu/integration:
mailbox: Introduce Qualcomm APCS IPC driver
dt-bindings: mailbox: Introduce Qualcomm APCS global binding
mailbox: Make startup and shutdown ops optional