ppc4xx: POST UART: Use in/out_8() io-accessor functions

This patch fixes a problem in the PPC4xx POST UART driver. This driver
incorrectly used the in/out8() io-accessor functions. This could lead to
problems since these functions don't guarantee execution ordering. This
patch now replaces these functions with the correct ones.

Additionally the driver is converted to using the NS16550 struct instead
of macros for the register offsets.

And some common code is factored out for better maintainability.

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