ppc4xx: Rework of 4xx serial driver (4)

Change 4xx_uart.c:

- Use in_8/out_8 macros instead of in8/out8
- No need for UART_BASE marco anymore, now really handled via function
  parameter
- serial_init_common() introduced
- Further coding style cleanup

Signed-off-by: Stefan Roese <sr@denx.de>