ppc4xx: Rework of 4xx serial driver (1)

This patch starts the rework of the PPC4xx serial driver. First we split
the file into two seperate files, one 4xx_uart.c with the 405/440 UART
handling code and the other one iop480_uart.c with the UART code for the
PLX-Tech IOP480 PPC (PPC403 based).

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