tty: serial: 8250: Add Mediatek UART driver

The device has a highspeed register which influences the calcualtion
of the divisor. The chip lacks support for some baudrates. When requested,
we set the divisor to the next smaller baudrate and adjust the c_cflag
accordingly.

Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>
Reviewed-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --cc drivers/tty/serial/8250/8250_mtk.c
Simple merge

diff --cc drivers/tty/serial/8250/Kconfig
Simple merge

diff --cc drivers/tty/serial/8250/Makefile
Simple merge