ppc4xx: TLB init file cleanup

This patch adds new macros, with frequently used combinations of the
4xx TLB access control and storage attibutes. Additionally the 4xx init.S
files are updated to make use of these new macros. Resulting in easier
to read TLB definitions.

Additionally some init.S files are updated to use the mmu header for the
TLB defines, instead of defining their own macros.

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