ppc4xx: Rework of 4xx serial driver (2)

Change all linker scripts to reference the changed driver name 4xx_uart.o.

Note: In most cased all these explicit referencing of these object files
in the linker scripts is not neccessary. Only for manually embedded
environment into the U-Boot image, which is not done is most cases.

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