powerpc: Fix build issue with CONFIG_RELOCATABLE=y

There are two issues when we enable CONFIG_RELOCATABLE.  The first is due
to the fact that phys_addr_t is now defined in linux/types.h.  The second
is due to the fact that the DMA code changes expose memstart_addr to
prom_init.c

Signed-off-by: Kumar Gala <galak@kernel.crashing.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>