serial: 8250_ingenic: Enable hardware flow control

The Ingenic UART is similar to a standard 16550, but hardware flow control
requires setting a couple of additional, non-standard bits in the MCR.

The non-standard "modem control enable" and "hardware flow control
mode" bits are set when writing to the MCR register, based
on whether the modem control interrupt is active.

Additionally the non-16550 compliant parts of the uart need to be
masked from higher layers.

Tested on Ingenic JZ4780 on MIPS Creator Ci20 board

Signed-off-by: Matt Redfearn <matt.redfearn@imgtec.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>