serial: pxa: Implement default_serial_console in serial_pxa.c

Implement weak default_serial_console() function in serial_pxa
driver, which gets available in case CONFIG_SERIAL_MULTI is
enabled. This will get helpful in subsequent patches, which will
encapsulate PXA's struct serial_device instances in serial_pxa
driver.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Marek Vasut <marek.vasut@gmail.com>
Cc: Tom Rini <trini@ti.com>