[POWERPC] 4xx: Fix duplicate phys_addr_t definition

Commit d04ceb3f moved phys_addr_t
definitions to include/asm-powerpc/types.h.  However, arch/ppc 440
builds had a duplicate definition in include/asm-ppc/mmu.h that caused
the build to fail.

This removes the duplicate definition in arch/ppc.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>