printk: add infrastucture for atomic consoles

Many times it is not possible to see the console output on
panic because printing threads cannot be scheduled and/or the
console is already taken and forcibly overtaking/busting the
locks does provide the hoped results.

Introduce a new infrastructure to support "atomic consoles".
A new optional callback in struct console, write_atomic(), is
available for consoles to provide an implemention for writing
console messages. The implementation must be NMI safe if they
can run on an architecture where NMIs exist.

Console drivers implementing the write_atomic() callback must
also select CONFIG_HAVE_ATOMIC_CONSOLE in order to enable the
atomic console code within the printk subsystem.

If atomic consoles are available, panic() will flush the kernel
log only to the atomic consoles (before busting spinlocks).
Afterwards, panic() will continue  as before, which includes
attempting to flush the other (non-atomic) consoles.

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