LoongArch: Add ARCH_HAS_DEBUG_VM_PGTABLE support

Add ARCH_HAS_DEBUG_VM_PGTABLE selection in Kconfig, in order to make
corresponding vm debug features usable on LoongArch. Also update the
corresponding arch-support.txt document.
Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>

Documentation/features/debug/debug-vm-pgtable/arch-support.txt

@@ -12,7 +12,7 @@
     |       arm64: |  ok  |
     |        csky: | TODO |
     |     hexagon: | TODO |
-    |   loongarch: | TODO |
+    |   loongarch: |  ok  |
     |        m68k: | TODO |
     |  microblaze: | TODO |
     |        mips: | TODO |

arch/loongarch/Kconfig

@@ -16,6 +16,7 @@ config LOONGARCH
 	select ARCH_HAS_ACPI_TABLE_UPGRADE	if ACPI
 	select ARCH_HAS_CPU_FINALIZE_INIT
 	select ARCH_HAS_CURRENT_STACK_POINTER
+	select ARCH_HAS_DEBUG_VM_PGTABLE
 	select ARCH_HAS_FAST_MULTIPLIER
 	select ARCH_HAS_FORTIFY_SOURCE
 	select ARCH_HAS_KCOV