ARM: dts: meson: add a node which describes the SRAM

All 32bit Meson SoCs contain 128KiB SRAM. This SRAM is used when
suspending the device (the the ARM Power Firmware on
Meson8/Meson8b/Meson8m2 saves the DDR settings there) and to boot the
secondary CPU cores.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>