ARM: dts: imx51: Fix sram.yaml warnings

Add ranges, #address-cells and #size-cells properties to the sram
node to fix the following warnings when checking sram.yaml:

make dtbs_check DT_SCHEMA_FILES=sram.yaml
...
arch/arm/boot/dts/imx51-apf51.dtb: sram@1ffe0000: '#address-cells' is a required property
	From schema: Documentation/devicetree/bindings/sram/sram.yaml
arch/arm/boot/dts/imx51-apf51.dtb: sram@1ffe0000: '#size-cells' is a required property
	From schema: Documentation/devicetree/bindings/sram/sram.yaml
arch/arm/boot/dts/imx51-apf51.dtb: sram@1ffe0000: 'ranges' is a required property
	From schema: Documentation/devicetree/bindings/sram/sram.yaml

Signed-off-by: Fabio Estevam <festevam@denx.de>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>