Implement the following apis to meet usage in different scenarios.

 - ioremap          (NonCache + StrongOrder)
 - ioremap_nocache  (NonCache + StrongOrder)
 - ioremap_wc       (NonCache + WeakOrder  )
 - ioremap_cache    (   Cache + WeakOrder  )

Also change flag VM_ALLOC to VM_IOREMAP in get_vm_area_caller.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Christoph Hellwig <hch@infradead.org>