ARM: dts: sun7i: Add A20 SRAM and SRAM controller

The A20 has a few SRAM that can be mapped either to a device or to the CPU,
with the mapping being controlled by a SRAM controller.

Since most of the time these SRAM won't be accessible by the CPU,
we can't use the mmio-sram driver and compatible.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
[hdegoede@redhat.com: Do not change soc node name, change compatible to
 sun4i-a10-sram-controller to match the driver change]
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>