lantiq: Make the ar9.dtsi sram node match "simple-bus"

lantiq: Make the ar9.dtsi sram node match "simple-bus"

All other SoC types are using "lantiq,sram" and "simple-bus" to ensure
that all child nodes are set up correctly during linux kernel
initialization (plat_of_setup(void) in arch/mips/lantiq/prom.c). Without
this some of sram child nodes might not be parsed.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
git-svn-id: svn://svn.openwrt.org/openwrt/trunk@48548 3c298f89-4303-0410-b956-a3cf2f4a3e73