[riscv] Return virtual address offset from enable_paging()

[riscv] Return virtual address offset from enable_paging()

Once paging has been enabled, there is no direct way to determine the
virtual address offset without external knowledge.  (The paging mode,
if needed, can be read directly from the SATP CSR.)

Change the return value from enable_paging() to provide the virtual
address offset.

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