serial: 8250_pxa: hide early console setup when disabled

The newly added pxa glue driver for 8250 supports console output, but
fails to build if the 8250 console is disabled:

drivers/tty/serial/8250/8250_pxa.o: In function `early_serial_pxa_setup':
8250_pxa.c:(.init.text+0x50): undefined reference to `early_serial8250_setup'

This adds an #ifdef like the other glue drivers have it.

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>