powerpc/40x: Add support for PPC40x boards with > 512MB SDRAM

This patch adds support for boards with more that 512MByte RAM. Currently
only 512MB of memory are enabled in the DCCR/ICCR real-mode cache
control registers. This patch now enables caching in real-mode for
2GByte.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>