serial: amba-pl011: Implement nbcon console

Implement the callbacks required for an NBCON console [0] on the
amba-pl011 console driver.

Referred to the NBCON implementation work for 8250 [1] and imx [2].

The normal-priority write_thread checks for console ownership
each time a character is printed.
write_atomic holds the console ownership until the entire string
is printed.

UART register operations are protected from other contexts by
uart_port_lock, except for a final flush(nbcon_atomic_flush_unsafe)
on panic.

The patch has been verified to correctly handle the output and
competition of messages with different priorities and flushing
panic message to console after nmi panic using ARM64 QEMU and
a physical machine(A64FX).

Stress testing was conducted on a physical machine(A64FX).
The results are as follows:

- The output speed and volume of messages using the NBCON console were
  comparable to the legacy console (data suggests a slight improvement).

- When inducing a panic (sysrq-triggered or NMI) under heavy contention
  on the serial console output,
  the legacy console resulted in the loss of some or all crash messages.
  However, using the NBCON console, no message loss occurred.

This testing referenced the NBCON console work for 8250 [3].

[0] https://lore.kernel.org/all/ZuRRTbapH0DCj334@pathway.suse.cz/
[1] https://lore.kernel.org/all/20240913140538.221708-1-john.ogness@linutronix.de/T/
[2] https://lore.kernel.org/linux-arm-kernel/20240913-serial-imx-nbcon-v3-1-4c627302335b@geanix.com/T/
[3] https://lore.kernel.org/lkml/ZsdoD6PomBRsB-ow@debarbos-thinkpadt14sgen2i.remote.csb/#t



Signed-off-by: Toshiyuki Sato <fj6611ie@aa.jp.fujitsu.com>
Link: https://lore.kernel.org/r/20250204044428.2191983-1-fj6611ie@aa.jp.fujitsu.com


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>