[POWERPC] Include <asm/mmu.h> in arch/powerpc/sysdev/fsl_soc.h for phys_addr_t.

This patch causes fsl_soc.h to import the definition of phys_addr_t
itself, rather than relying on its includer to do so.

Signed-off-by: Scott Wood <scott@freescale.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>