riscv: Select ARCH_USE_MEMTEST

As of commit dce44566

 ("mm/memtest: add ARCH_USE_MEMTEST"),
architectures must select ARCH_USE_MEMTESET to enable CONFIG_MEMTEST.

Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Fixes: f6e5aedf

 ("riscv: Add support for memtest")
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>