tty: serial: allow pxa.c to be COMPILE_TESTed

There is no issue compiling pxa.c even in the SERIAL_8250=y case. So to
cover it in the usual configurations, add "|| COMPILE_TEST" there.

Signed-off-by: Jiri Slaby <jslaby@suse.cz>
Link: https://lore.kernel.org/r/20220927110528.12815-1-jslaby@suse.cz


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