serial: pxa2xx: mark PM functions as __maybe_unused

The fresh new serial driver for pxa produces warnings when
CONFIG_PM_SLEEP is disabled:

drivers/tty/serial/8250/8250_pxa.c:50:12: error: 'serial_pxa_resume' defined but not used [-Werror=unused-function]
drivers/tty/serial/8250/8250_pxa.c:41:12: error: 'serial_pxa_suspend' defined but not used [-Werror=unused-function]

This removes the #ifdef around the two functions and instead marks both
as __maybe_unused, which is more robust and avoids the warning.

Fixes: ab28f51c ("serial: rewrite pxa2xx-uart to use 8250_core")
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Robert Jarzmik <robert.jarzmik@free.fr>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>