serial: 8250_pci: add support for Intel BayTrail

Intel BayTrail has two HS-UARTs with 64 byte fifo, support
for DMA and support for 16750 compatible Auto Flow Control.

Signed-off-by: Heikki Krogerus <heikki.krogerus@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>