mfd: intel-lpss: Use writeq() helper

There are already helper functions to do 64-bit I/O on 32-bit machines, thus we
don't need to reinvent the wheel. In our case we can't use readq() / writeq()
even on 64-bit kernel since there is a hardware limitation (OCP bus is a 32-bit
bus).

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>