[riscv] Return accessible physical address space size from enable_paging()

[riscv] Return accessible physical address space size from enable_paging()

Relocation requires knowledge of the size of the accessible physical
address space, which for 64-bit CPUs will vary according to the paging
level supported by the processor.

Update enable_paging_64() and enable_paging_32() to calculate and
return the size of the accessible physical address space.

Signed-off-by: Michael Brown <mcb30@ipxe.org>