mbox: bcm2835: Change mailbox channel DTS config to sub nodes

Signed-off-by: Craig McGeachie <slapdau@yahoo.com.au>
Revert "mbox: Move mailbox build to point before other devices which will depend"

This reverts commit 74ae9daf6251710fc6fa3fb46dce3b6e1445ba03.
Merge branch 'master' into topic/thermal
bcm2835: thermal: Implement probe deferral to wait for mailbox.

Signed-off-by: Craig McGeachie <slapdau@yahoo.com.au>