Dualcpu: Rename MAILBOX_SIGNAL to MAILBOX_INTERRUPT_SIGNAL

Rename the Mailbox signal name to align with Mailbox NS Agent
update document.

Change-Id: I1521e7f62a44d9aaa3aa681508f871ea85784297
Signed-off-by: Jianliang Shen <jianliang.shen@arm.com>
5 files changed