mips: msp71xx/serial: add workaround for DW UART

The Synopsys DesignWare UART in pmc-sierra msp71xx has an extra feature
where the UART detects a write attempt to the LCR whilst busy and raises
an interrupt.  The driver needs to clear the interrupt and rewrite the
LCR.  Move this into platform code and out of the 8250 driver.

Acked-by: Ralf Baechle <ralf@linux-mips.org>
Acked-by: Alan Cox <alan@linux.intel.com>
Signed-off-by: Jamie Iles <jamie@jamieiles.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>