riscv: Add support to dump the kernel page tables

In a similar manner to arm64, x86, powerpc, etc., it can traverse all
page tables, and dump the page table layout with the memory types and
permissions.

Add a debugfs file at /sys/kernel/debug/kernel_page_tables to export
the page table layout to userspace.

Signed-off-by: Zong Li <zong.li@sifive.com>
Tested-by: Alexandre Ghiti <alex@ghiti.fr>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>