misc: sram: document new compatible

Add atmel,sama5d2-securam to the compatible list.
Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Acked-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/sram/sram.txt

@@ -4,7 +4,7 @@ Simple IO memory regions to be managed by the genalloc API.
 
 Required properties:
 
-- compatible : mmio-sram
+- compatible : mmio-sram or atmel,sama5d2-securam
 
 - reg : SRAM iomem address range