RISC-V: relocate DTB if it's outside memory region

In case the DTB provided by the bootloader/BootROM is before the kernel
image or outside /memory, we won't be able to access it through the
linear mapping, and get a segfault on setup_arch(). Currently OpenSBI
relocates DTB but that's not always the case (e.g. if FW_JUMP_FDT_ADDR
is not specified), and it's also not the most portable approach since
the default FW_JUMP_FDT_ADDR of the generic platform relocates the DTB
at a specific offset that may not be available. To avoid this situation
copy DTB so that it's visible through the linear mapping.

Signed-off-by: Nick Kossifidis <mick@ics.forth.gr>
Link: https://lore.kernel.org/r/20220322132839.3653682-1-mick@ics.forth.gr
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Fixes: f105aa94

 ("riscv: add BUILTIN_DTB support for MMU-enabled targets")
Cc: stable@vger.kernel.org
Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>