[riscv] Construct invariant portions of page table outside the loop

[riscv] Construct invariant portions of page table outside the loop

The page table entries for the identity map vary according to the
paging level in use, and so must be constructed within the loop used
to detect the maximum supported paging level.  Other page table
entries are invariant between paging levels, and so may be constructed
just once before entering the loop.

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