rockchip: rk3288: sdram: fix DDR address range

The all current Rockchip SoCs supporting 4GB of ram have problems
accessing the memory region 0xfe000000~0xff000000. Actually, some IP
controller can't address to, so let's limit the available range.

This patch fixes a bug which found in miniarm-rk3288-4GB board. The
U-Boot was relocated to 0xfef72000, and .bss variants was also
relocated, such as do_fat_read_at_block. Once eMMC controller transfer
data to do_fat_read_at_block via DMA, DMAC can't access more than
0xfe000000. So that DMAC didn't work sane.

Signed-off-by: Ziyuan Xu <xzy.xu@rock-chips.com>
Acked-by: Simon Glass <sjg@chromium.org>
Tested-by: Simon Glass <sjg@chromium.org>