ARM: Meson6: Add SMP support for Amlogic Meson6

Amlogic Meson6 is a dual core Cortex-A9. This patch adds the logic to
boot up the second CPU.

Signed-off-by: Carlo Caione <carlo@caione.org>
ARM: Meson6: enable SMP in DTS/DTSI

Signed-off-by: Carlo Caione <carlo@caione.org>