pch_uart: add multi-scatter processing

Currently, this driver can handle only single scatterlist.
Thus, it can't send data beyond FIFO size.

This patch enables this driver can handle multiple scatter list.

Signed-off-by: Tomoya MORINAGA <tomoya-linux@dsn.okisemi.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>