x86: Allow I/O functions to use pointers

x86: Allow I/O functions to use pointers

It is common with memory-mapped I/O to use the address of a structure member
to access memory, as in:

   struct some_regs {
      u32 ctrl;
      u32 data;
   }

   struct some_regs *regs = (struct some_regs *)BASE_ADDRESS;

   writel(1, &reg->ctrl);
   writel(2, &reg->data);

This does not currently work with inl(), outl(), etc. Add a cast to permit
this.

Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>