RISC-V: Place DTB at 3GB boundary instead of 4GB

RISC-V: Place DTB at 3GB boundary instead of 4GB

Currently, we place the DTB at 2MB from 4GB or end of DRAM which ever is
lesser. However, Linux kernel can address only 1GB of memory for RV32.
Thus, it can not map anything beyond 3GB (assuming 2GB is the starting address).
As a result, it can not process DT and panic if opensbi dynamic firmware
is used. While at it, place the DTB further away to avoid in memory placement
issues in future.

Fix this by placing the DTB at 16MB from 3GB or end of DRAM whichever is lower.

Fixes: 66b1205bc5ab ("RISC-V: Copy the fdt in dram instead of ROM")

Reviewed-by: Bin Meng <bin.meng@windriver.com>
Tested-by: Bin Meng <bin.meng@windriver.com>
Signed-off-by: Atish Patra <atish.patra@wdc.com>
Message-id: 20210107091127.3407870-1-atish.patra@wdc.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>