sdram base address requirement for nios2-linux

The low memory, which kernel can access without TLB, is 0x0-0x1fffffff physical address (512MB span). Divide this by 2 gives 256MB. In order to run a single kernel image on different hardware using fdt, we should setup a hardware design guide line that sdram base should be at 0x10000000 physical address. All other components, such as flash, serial and ethernet, should be placed in 0x00000000-0x0fffffff (256MB span).

- Hippo