refactor(ast2700): adopt RESET_TO_BL31 boot flow

Revise the AST2700 boot flow to the RESET_TO_BL31 scheme.
The execution of BL1/2 can be saved from ARM CA35 while most
low level platform initialization are moved to a preceding MCU.

This patch updates the build configuration and also adds
the SMP mailbox setup code to hold secondary cores until
they are being waken up.

Signed-off-by: Chia-Wei Wang <chiawei_wang@aspeedtech.com>
Change-Id: I7e0aa6416b92b97036153db1d9a26baaa41b7b18
5 files changed