serial: 8250: implement non-BKL console

Implement the necessary callbacks to allow the 8250 console driver
to perform as a non-BKL console. Remove the implementation for the
legacy console callback (write) and add implementations for the
non-BKL consoles (write_atomic, write_thread, port_lock) and add
CON_NO_BKL to the initial flags.

Although non-BKL consoles can coexist with legacy consoles, you
will only receive all the benefits of the non-BKL consoles if
this console driver is the only console. That means no netconsole,
no tty1, no earlyprintk, no earlycon. Just the uart8250.

For example: console=ttyS0,115200

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