riscv: hmp: Add a command to show virtual memory mappings

This adds 'info mem' command for RISC-V, to show virtual memory
mappings that aids debugging.

Rather than showing every valid PTE, the command compacts the
output by merging all contiguous physical address mappings into
one block and only shows the merged block mapping details.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
Reviewed-by: Palmer Dabbelt <palmer@sifive.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>