ppc4xx: Use common ns16550 functions in 4xx UART POST driver

This patch changes the PPC4xx POST UART driver to use the common
NS16550 functions for receiving and sending. Additionally the
local function for SoC divisor setup are removed. Instead the
functions from arch/powerpc/cpu/ppc4xx/4xx_uart.c are used. This
removes code duplication.

Also the common CONFIG_SYS_NS16550_COMx defines are now used
to describe the POST UART's.

And a compile breakage is fixed, introduced by a git merge of
the ppc4xx/next branch into master. Now "ppc4xx.h" is moved to
"asm/ppc4xx.h". Fixed as well with this patch.

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