ARM: dts: sun9i: Add secure SRAM node used for SMP hotplug

The A80 stores some magic flags in a portion of the secure SRAM. The
BROM jumps directly to the software entry point set by the SMP code
if the flags are set. This is required for CPU0 hotplugging.

Acked-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Signed-off-by: Chen-Yu Tsai <wens@csie.org>