RISC-V: Minimal QEMU 2.12 fix for sifive_u machine

RISC-V: Minimal QEMU 2.12 fix for sifive_u machine

The 'sifive_u' board has a bug where the ROM is
created as RAM at the wrong address and marked
readonly. The bug renders the board unusable.
This is a minimal fix and allows booting Linux.

5aec3247c190f10654250203a1742490ae7343a2
"RISC-V: Mark ROM read-only after copying in code"
contains a comprehensive fix using the ROM APIs
memory_region_init_rom and rom_add_blob_fixed_as
which could be backported.

Cc: Sagar Karandikar <sagark@eecs.berkeley.edu>
Cc: Bastian Koppelmann <kbastian@mail.uni-paderborn.de>
Cc: Palmer Dabbelt <palmer@sifive.com>
Cc: Alistair Francis <Alistair.Francis@wdc.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Michael Clark <mjc@sifive.com>
Signed-off-by: Michael Roth <mdroth@linux.vnet.ibm.com>