ARM: orion: fix include path

Now that CONFIG_ARCH_MULTIPLATFORM can be disabled anywhere,
there is a build failure for plat-orion:

arch/arm/plat-orion/irq.c:19:10: fatal error: plat/irq.h: No such file or directory

Make the include path unconditional.

Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>