hw/riscv: Drop 'struct MemmapEntry'

hw/riscv: Drop 'struct MemmapEntry'

There is already a MemMapEntry type defined in hwaddr.h. Let's drop
the RISC-V defined `struct MemmapEntry` and use the existing one.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-id: 20210220144807.819-2-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>