x86: Implement arch-specific io accessor routines

At present the generic io{read,write}{8,16,32} routines only support
MMIO access. With architecture like x86 that has a separate IO space,
these routines cannot be used to access I/O ports.

Implement x86-specific version to support both PIO and MMIO access,
so that drivers for multiple architectures can use these accessors
without the need to know whether it's MMIO or PIO.

These are ported from Linux kernel lib/iomap.c, with slight changes.

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