serial: 8250: implement write_atomic

Implement a non-sleeping NMI-safe write_atomic() console function in
order to support atomic console printing during a panic.

Trasmitting data requires disabling interrupts. Since write_atomic()
can be called from any context, it may be called while another CPU
is executing in console code. In order to maintain the correct state
of the IER register, use the global cpu_sync to synchronize all
access to the IER register. This synchronization is only necessary
for serial ports that are being used as consoles.

The global cpu_sync is also used to synchronize between the write()
and write_atomic() callbacks. write() synchronizes per character,
write_atomic() synchronizes per line.

Signed-off-by: John Ogness <john.ogness@linutronix.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>