LoongArch: Select ARCH_USE_MEMTEST

LoongArch: Select ARCH_USE_MEMTEST

[ Upstream commit fb8e9f59d6f292c3d9fea6c155c22ea5fc3053ab ]

As of commit dce44566192e ("mm/memtest: add ARCH_USE_MEMTEST"),
architectures must select ARCH_USE_MEMTESET to enable CONFIG_MEMTEST.

Commit 628c3bb40e9a ("LoongArch: Add boot and setup routines") added
support for early_memtest but did not select ARCH_USE_MEMTESET.

Fixes: 628c3bb40e9a ("LoongArch: Add boot and setup routines")
Tested-by: Erpeng Xu <xuerpeng@uniontech.com>
Tested-by: Yuli Wang <wangyuli@uniontech.com>
Signed-off-by: Yuli Wang <wangyuli@uniontech.com>
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
Signed-off-by: Sasha Levin <sashal@kernel.org>