Doc: Update out-dated info about Juno's mailbox
Doc: Update out-dated info about Juno's mailbox

Since commit 804040d106, the Juno port has moved from per-CPU mailboxes
to a single shared one. This patch updates an out-dated reference to
the former per-CPU mailboxes mechanism in the Firmware Design.

Change-Id: I355b54156b1ace1b3df4c4416e1e8625211677fc

lsigithub/arm-trusted-firmware_public,sandrine-bailleux/arm-trusted-firmware,davwan01/arm-trusted-firmware,sandrine-bailleux/arm-trusted-firmware,sandrine-bailleux-arm/arm-trusted-firmware,achingupta/arm-trusted-firmware,sandrine-bailleux-arm/arm-trusted-firmware,achingupta/arm-trusted-firmware,lsigithub/arm-trusted-firmware_public,davwan01/arm-trusted-firmware,sbranden/arm-trusted-firmware,sbranden/arm-trusted-firmware