RISC-V: load initrd wherever it fits into memory

Requiring that initrd is loaded below RAM start + 256 MiB led to failure
to boot SUSE Linux with GRUB on QEMU, cf.
https://lists.gnu.org/archive/html/grub-devel/2021-06/msg00037.html



Remove the constraint.

Reported-by: Andreas Schwab <schwab@linux-m68k.org>
Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Fixes: d7071743

 ("RISC-V: Add EFI stub support.")
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>