[POWERPC] ppc: Fix PCIX configuration of Ocotea & Taishan for > 512MB DDR

Change the configuration of the PCIX PCI->PLB inbound memory window
to be 2GB instead of 512kB. The comment already mentioned 2GB, but the
code unfortunately didn't reflect this.

Signed-off-by: Stefan Roese <sr@denx.de>
Signed-off-by: Paul Mackerras <paulus@samba.org>