documentation: Add secure monitor bindings documentation

Add the binding documentation for the Amlogic secure monitor driver.
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NCarlo Caione <carlo@endlessm.com>
Signed-off-by: NKevin Hilman <khilman@baylibre.com>

Documentation/devicetree/bindings/firmware/meson/meson_sm.txt

+* Amlogic Secure Monitor
+
+In the Amlogic SoCs the Secure Monitor code is used to provide access to the
+NVMEM, enable JTAG, set USB boot, etc...
+
+Required properties for the secure monitor node:
+- compatible: Should be "amlogic,meson-gxbb-sm"
+
+Example:
+
+	firmware {
+		sm: secure-monitor {
+			compatible = "amlogic,meson-gxbb-sm";
+		};
+	};