powerpc: Change BAT code to use phys_addr_t

Currently, the physical address is an unsigned long, but it should
be phys_addr_t in set_bat, [v/p]_mapped_by_bat.  Also, create a
macro that can convert a large physical address into the correct
format for programming the BAT registers.

Signed-off-by: Becky Bruce <becky.bruce@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>