parisc: serial/mux: Convert to uart_console_device instead of open-coded

The implementation of mux_console_device() is very similar to
uart_console_device(). Setting .data field in mux_console then we can
convert to use uart_console_device().

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Helge Deller <deller@gmx.de>