csky: bugfix tlb_get_pgd error.

csky: bugfix tlb_get_pgd error.

It's wrong to mask/unmask highest bit in addr to translate the vaddr
to paddr. We should use PAGE_OFFSET and PHYS_OFFSET.

Wrong implement:
  return ((get_pgd()|(1<<31)) - PHYS_OFFSET) & ~1;

When PHYS_OFFSET=0xc0000000 and get_pgd() return 0xe0000000, it'll
return 0x60000000. It's wrong and should be 0xa0000000.

Now correct it to:
  return ((get_pgd() - PHYS_OFFSET) & ~1) + PAGE_OFFSET;

Signed-off-by: Guo Ren <ren_guo@c-sky.com>