riscv: leaf page (2MB page)

https://www2.eecs.berkeley.edu/Pubs/TechRpts/2015/EECS-2015-49.pdf

Any level of PTE may be a leaf PTE, so in addition to 4 KiB pages, Sv39 supports 2 MiB megapages
and 1 GiB gigapages, each of which must be virtually and physically aligned to a boundary equal
to its size.

https://www.reddit.com/r/RISCV/comments/v895do/what_are_super_pages_wrt_page_tables/?tl=ko

crash> vtop 0000003fbe7c33f8
VIRTUAL           PHYSICAL
3fbe7c33f8        12c4d53f8

PAGE DIRECTORY: ffffffd8c09e4000
  PGD: ffffffd8c09e47f0 => 417f5801  // 0001 [non-leaf]
  PMD: 0000000105fd6000 => 000000004d32a801 // 0001 [non-leaf]
  PTE: 134caa000 => 4b1354d7
 PAGE: 000000012c4d5000

crash> vtop 0xffffffff813f0cb0
VIRTUAL           PHYSICAL
ffffffff813f0cb0  415f0cb0

PAGE DIRECTORY: ffffffff81c4f000
  PGD: ffffffff81c4fff0 => 4fffe801   // 0001 [non-leaf]
  PMD: 000000013fffa000 => 00000000105000e3  // 0011 [leaf], 2MB page
  PTE: 41400000 => ffffffff80a29ce2

     PTE         PAGE: 0000000000000000 not present