hw/riscv: sifive_e: Add 'const' to sifive_e_memmap[]

hw/riscv: sifive_e: Add 'const' to sifive_e_memmap[]

This was accidentally dropped before. Add it back.

Fixes: 732612856a8 ("hw/riscv: Drop 'struct MemmapEntry'")
Reported-by: Emmanuel Blot <eblot.ml@gmail.com>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210331103612.654261-1-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>