serial: mfd: add a module parameter for setting each port's working mode

The three identical uart ports can work either in DMA or PIO mode. Adding such
a module parameter "hsu_dma_enable" will enable user to chose working modes for
each port. If the mfd driver is built in kernel, adding a "mfd.hsu_dma_enable=x"
in kernel command line has the same effect.

Signed-off-by: Feng Tang <feng.tang@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>