Add console_stop() and console_start() methods so the serial drivers
can disable console output before suspending a port, and re-enable output
afterwards.

We also add locking to ensure that we synchronise with any in-progress
printk.
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>