MPSC serial driver tx locking

The MPSC serial driver assumes that interrupt is always on to pick up the
DMA transmit ops that aren't submitted while the DMA engine is active.
However when irqs are off for a period of time such as operations under
kernel crash dump console messages do not show up due to additional DMA ops
are being dropped.  This makes console writes to process through all the tx
DMAs queued up before submitting a new request.

Also, the current locking mechanism does not protect the hardware registers
and ring buffer when a printk is done during the serial write operations.
The additional per port transmit lock provides a finer granular locking and
protects registers being clobbered while printks are nested within UART
writes.

Signed-off-by: Dave Jiang <djiang@mvista.com>
Signed-off-by: Mark A. Greer <mgreer@mvista.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>