[POWERPC] powerpc: fix build error in rom.c

Add missing include in rom.c.  Fixes this build error when CONFIG_MTD=y:

  arch/powerpc/sysdev/rom.c:26: error: implicit declaration of function
  of_platform_device_create

Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>