RISC-V: Implement sparsemem

Implement sparsemem support for Risc-v which helps pave the
way for memory hotplug and eventually P2P support.

Introduce Kconfig options for virtual and physical address bits which
are used to calculate the size of the vmemmap and set the
MAX_PHYSMEM_BITS.

The vmemmap is located directly before the VMALLOC region and sized
such that we can allocate enough pages to populate all the virtual
address space in the system (similar to the way it's done in arm64).

During initialization, call memblocks_present() and sparse_init(),
and provide a stub for vmemmap_populate() (all of which is similar to
arm64).

[[email protected]: fixed pfn_valid, FIXADDR_TOP and fixed a bug
 rebasing onto v5.3]
Signed-off-by: Greentime Hu <[email protected]>
Signed-off-by: Logan Gunthorpe <[email protected]>
Reviewed-by: Palmer Dabbelt <[email protected]>
Reviewed-by: Christoph Hellwig <[email protected]>
Cc: Albert Ou <[email protected]>
Cc: Andrew Waterman <[email protected]>
Cc: Olof Johansson <[email protected]>
Cc: Michael Clark <[email protected]>
Cc: Rob Herring <[email protected]>
Cc: Zong Li <[email protected]>
Reviewed-by: Mike Rapoport <[email protected]>
[[email protected]: updated to apply; minor commit message
 reformat]
Signed-off-by: Paul Walmsley <[email protected]>

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