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>