meson/gxl: Initial port of Amlogic Meson S905x (GXL)

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>