powerpc: Fix problems with 32bit PPC's running with >= 4GB of RAM

This patch enables 32bit PPC's (with 36bit physical address space, e.g.
IBM/AMCC PPC44x) to run with >= 4GB of RAM. Mostly its just replacing types
(unsigned long -> phys_addr_t).

Tested on an AMCC Katmai with 4GB of DDR2.

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