serial: pxa: Make use of default_serial_console in serial_pxa

serial: pxa: Make use of default_serial_console in serial_pxa

Make use of the newly implemented weak default_serial_console in
the serial_pxa driver. This removes all reimplementations of this
function from board files.

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