x86: Add dma-mapping.h to architectural code

Some cross-platform drivers rely on this header present.
Make it so for x86.

It's just a copy'n'paste of arch/arm/include/asm/dma-mapping.h.

Suggested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>