x86: Add 64-bit memory-mapped I/O functions

Add readq() and writeq() definitions for x86.

Please note: in 32-bit code readq/writeq will generate two 32-bit
memory access instructions instead of one atomic 64-bit operation.

Signed-off-by: Ivan Gorinov <ivan.gorinov@intel.com>
Reviewed-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>