Before CPU enters standby state (wfi), the AP needs to signal the SCP
through PSCI mailbox.

Also at boot time the AP has to wait for the SCP to be ready before
sending the first scpi commands or it can crash.

Change-Id: Iacc99f5bec745ad71922c5ea07ca5b87088133b6
Signed-off-by: Remi Pommarel <repk@triplefau.lt>

The Amlogic Meson S905x is a SoC with a quad core Arm Cortex-A53 running
at 1.5Ghz. It also contains a Cortex-M3 used as SCP.

This port is a minimal implementation of BL31 capable of booting
mainline U-Boot and Linux:

- Partial SCPI support.
- Basic PSCI support (CPU_ON, SYSTEM_RESET, SYSTEM_OFF).
- GICv2 driver set up.
- Basic SIP services (read efuse data, enable/disable JTAG).

This port has been tested on a lepotato.

Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>
Signed-off-by: Remi Pommarel <repk@triplefau.lt>