x86: mmu: relax KERNEL_VM_OFFSET == SRAM_OFFSET

There was a restriction that KERNEL_VM_OFFSET must equal to
SRAM_OFFSET so that page directory pointer (PDP) or page
directory (PD) can be reused. This is not very practical in
real world due to various hardware designs, especially those
where SRAM is not aligned to PDP or PD. So rework those bits.

Signed-off-by: Daniel Leung <daniel.leung@intel.com>