kernel: mmu: restore address conversion functions

This reverts commit 7d32e9f9a588fd3ed17277fb88210d453f2c9864.

These functions are needed for linking kernel in virtual address
space when it differs from the physical address space.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>
Signed-off-by: Daniel Leung <daniel.leung@intel.com>