[POWERPC] Move phys_addr_t definition into asm/types.h

Moved phys_addr_t out of mmu-*.h and into asm/types.h so we can use it in
places that before would have caused recursive includes.

For example to use phys_addr_t in <asm/page.h> we would have included
<asm/mmu.h> which would have possibly included <asm/mmu-hash64.h> which
includes <asm/page.h>.  Wheeee recursive include.

CONFIG_PHYS_64BIT is a bit counterintuitive in light of ppc64 systems
and thus the config option is only used for ppc32 systems with >32-bit
physical addresses (44x, 85xx, 745x, etc.).

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>