[POWERPC] 4xx: Add PPC4xx L2-cache support (440GX)

This patch adds support for the 256k L2 cache found on some IBM/AMCC
4xx PPC's. It introduces a common 4xx SoC file (sysdev/ppc4xx_soc.c)
which currently "only" adds the L2 cache init code. Other common 4xx
stuff can be added later here.

The L2 cache handling code is a copy of Eugene's code in arch/ppc
with small modifications.

Tested on AMCC Taishan 440GX.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>