[POWERPC] 4xx: Workaround for CHIP_11 Errata

The PowerPC 440EP, 440GR, 440EPx, and 440GRx chips have an issue that
causes the PLB3-to-PLB4 bridge to wait indefinitely for transaction
requests that cross the end-of-memory-range boundary.  Since the DDR
controller only returns the valid portion of a read request, the bridge
will prevent other PLB masters from completing their transactions.

This implements the recommended workaround for this errata for chips that
use older versions of firmware that do not already handle it.  The last
4KiB of memory are hidden from the kernel to prevent the problem
transactions from occurring.

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