riscv: Disable ARCH_HAS_DEBUG_VIRTUAL if NOMMU

DEBUG_VIRTUAL should only used when MMU enabled, add the dependence.
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>

arch/riscv/Kconfig

@@ -54,7 +54,7 @@ config RISCV
 	select GENERIC_ARCH_TOPOLOGY if SMP
 	select ARCH_HAS_PTE_SPECIAL
 	select ARCH_HAS_MMIOWB
-	select ARCH_HAS_DEBUG_VIRTUAL
+	select ARCH_HAS_DEBUG_VIRTUAL if MMU
 	select HAVE_EBPF_JIT if MMU
 	select EDAC_SUPPORT
 	select ARCH_HAS_GIGANTIC_PAGE