ARM: dts: imx6sll: add missing properties for sram

All 3 properties are required by sram.yaml. Fixes the dtbs_check warning:
sram@900000: '#address-cells' is a required property
sram@900000: '#size-cells' is a required property
sram@900000: 'ranges' is a required property

Signed-off-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>