MPC512x: factor out common code

Now that we have 3 boards for the MPC512x it turns out that they all
use the very same fixed_sdram() code.

This patch factors out this common code into cpu/mpc512x/fixed_sdram.c
and adds a new header file, include/asm-ppc/mpc512x.h, with some
macros, inline functions and prototype definitions specific to MPC512x
systems.

Signed-off-by: Wolfgang Denk <wd@denx.de>
Cc: Stefan Roese <sr@denx.de>
Cc: Reinhard Arlt <reinhard.arlt@esd-electronics.com>